It's not fear, it's simply unnecessary fragmentation. Let GOG do there thing, and improve on a basis for everyone at the same time. We don't need open source for that to happen.

To explain it to somebody like me (who has no clue) - there could be something the "Barefoot Essentials for GOG" for the client, modifying it in some ways? Or did I misunderstand that?

It's a concern without valid basis. Opening the client won't hurt GOG and will only benefit them and their users. On the contrary, not opening it would reduce trust in such client. GOG kind of already anticipate that - see their talk in the video where they say that if people are worried, they shouldn't be since the client is optional. They can address that better by opening the client itself.

Having users on multiple configurations while trying to support and secure those users is a valid concern.. a concern that every company ever has considered when making there software open or closed. What if someone a change to there client ( without realizing it) that conflicts with a game or games and potentially breaks or hinders a game... that person would come to GOG for support which which would make GOG's job more difficult if there using a modified client to find the cause even possibly costing them money if GOG thinks it's an issue with the game and refunds them.

What if a developer decides to make a modified client of GOG software and prioritize there games on that modified client over the others because it has features that GOG's doesn't.

It's not so black and white as you would like to make it...

If gog are wise they will keep the client in-house until they have a solid stable version that works without any issues and then make it open-source down the years if they feel they have done enough standard features to the client.

doing it now right at release is like opening the floodgates . They will want to work it on their own, get it popular and then do stuff for the modders

Since it's open, if something breaks it can be fixed. That's the point after all. GOG won't "support" other clients. They support theirs. Protocol should handle errors properly. So erroneous clients won't work and won't break anything on the server.

Your concerns are rather imaginary, really without some actual examples. In practice it works all around. Look at any open protocol (HTTP, IMAP, SMTP, XMPP, Bittorrent and etc.). They are all open and have tons of clients (and even servers) to choose from. And they interoperate precisely because they are open.

They can release it when they are ready. But if they will propose it for users to use, we can assume they think it's ready. It should be opened at the same time ideally. And what did you meany by floodgates? Being overwhelmed with bug fixes and community pull requests? I don't think GOG worry about that. They'd actually probably like it.

Your missing my point... rather if it can be fixed or not is irrelevant. Errors when coding happen, sometimes without being caught and without actually being able to tell something is broken. Sometimes with open source projects things are changed from how they were meant to work. When this happens all your average joe GOG gamer is going to know is it doesn't work or doesn't work right and will come to GOG for support. GOG will have to offer them support because of there money back guarantee and because that is what they do without really knowing if it's a game issue or client issue. Of course GOG won't support unofficial clients... but when if and unofficial client effects a game it can be a pain to figure that out.

Your trying to use other open source projects to justify it... when talking about something specific here. A gaming client and how fragmentation in theory could effect it.

Again unnecessary fragmentation... simply because you want to know what GOG is doing in there program that is being provided. Something we really don't need to know. I trust GOG, so for me this isn't an issue.

I can see that more than starting out with it being open source.

It's still speculating about clients multiplying to a degree that GOG will be overwhelmed with errors in the forks. Who said it will even happen. Fear of such forks is irrational, I'll just repeat that. May be you can show a practical example of such problem, because I'm not aware of any.

Meanwhile GPL source ports and games can't support GOG Galaxy.

You are aware that Open Source doesn't mean that anyone can modify it, right? You are free to grab the source, modify it as you like, but you cannot commit your changes to the main repository.

So you'll have "GOG Galaxy" available from GOG.com only, and "JMich's Galaxy" available from my OneDrive. If you use my galaxy and ask GOG for support, their first step will be to direct you to their own client and ask you to re-install it. Or you can ask me to support, in which case I reply that the client is provided "as-is".

If my client covers your needs, then you can use it, if it doesn't cover them, you ignore it. But assuming proper coding, it won't create fragmentation. See GOG Downloader and lgogdownloader for an example.

Why? It would depend on the provided API, and how much access you need in it.

For the same reason they can't use Steamworks as is. You have use GPL incompatibly licensed libraries. If instead GOG were to distribute GOG Galaxy(including source code) under the MIT license this wouldn't be a problem.

Haven't really checked the GPL license, but what do you mean "incompatibility licensed libraries"? Assume that the client side API is available (packets require to be formatted as X, send to Y address) but the server side isn't available. Doesn't that cover GPL? If you send the server the packets it expects, it parses them and responds, if you send different packets, it ignores them.

I know that... but people would no doubt share there creations here... and average joe thinking GOG Galaxy Awesome Edition posted in the forum is cool, downloads... has a small issue were a game won't install or play for whatever reason and lets say it was caused by a client error. Thinking it's the game contacts GOG support, GOG doesn't ask about a unofficial client being used or average joe doesn't tell GOG... support get's caught up trying to fix an issue caused by simply having an unofficial client.

I'm not saying this type of thing would happen a lot or even happen at all... simply that trying to support and keep your client secure is challenging enough without people tearing it apart, finding loop holes in the code and sharing them for anyone to take advantage of. I just find it unnecessary make it open source, at-least for right now at this early in the game.

Down the road, may be a different...

I am not sure I am following you. Are you talking about say a Quake 3 Arena source port doing its own implementation of Galaxy? That is not what I am talking about. I am talking about them using Galaxy AS provided by GOG. However even in the former case would (ideally) require GOG to release the APIs and protocols out in the open.

First step of supporting

"Try reinstalling. You can find the software here"
"What version of our software are you using? Do X and send us the log file"

First steps which ensure that you are not supporting something you shouldn't be. And GOG support already does that, there have been answers similar to "We are sorry, we don't currently offer support for that platform" from support.
To connect to Galaxy, I assume you need two parts. A client part and a server part. The client part is available, and you can add it to your program. The server part is not available, and you don't know what it does. You do know its responses though, and how it forwards the packets it receives.
Can you add that to a GPL license? Even if one half of the code is unknown to you?