Is there anything major I would be missing out on if I bought Dead Cells on GOG instead of Steam?

I'd like to note that I will be using a Logitech controller. I am not sure how well GOG games do with generic game controllers?

Thanks

On Steam you have access to dev versions, on GOG only to stable versions. So it depends if you prefer novelty or stability.

On Steam you have modding tools that are missing on GOG.

For the controller, I don't know but I see any reason to have it working on one plateform and not the other.

If you are a Linux user and bought on GOG you miss out on daily runs and twitch integration.

well i'm a windows user and bought it on GOG and i cannot see how to make twitch integration to work either... (didnt try the daily runs but i suspect it won't work either ?)

Oh really? I was assuming those things work, provided you use Galaxy.

ah that's it, i dont use galaxy... though i didnt know there was some "link twitch account" parameter ing alaxy... and i already played other games (Party Hard, Hand of Fate 2...) which did the job perfectly fine without the need of the "optional" client

Yes, it's perfectly possible without the need for such a client. It's a minor convenience feature they offer there as the users don't need to generate and copy/paste the OAUTH token themselves.
I'd be even fine with the Slay the Spire approach, which means no UI at all, just put the info into a config file.

Just another note, if you play on Steam you have access to the Steam Input Configurator, which will allow you to use the Steam Controller (which I already do) and give you control over other aspects of the game which the in-game controller config won't give you (like customised speed curves for joysticks). If you play the game with a dpad rather than a joystick, or you use a keyboard+mouse this isn't really relevant, although being able to do things like have extra actions on button double-press etc is potentially useful.

In general, the Steam version is better since it has Alpha/Beta access, the devs are very active on the Steam community forum, and it has workshop support. I really hope we get GOG Connect soon, as I love GOG a lot and generally prefer the platform over Steam, but the Steam version is just objectively better unfortunately

The steam controller config tool should not be underestimated. Unlike, say, Timespinner, Dead Cells has no way of reconfiguring its controls to be anything but an XBox controller. So if you use a different input device then at the very least your on-screen button prompts won't line up with the actual buttons.

Case in point I got an 8bitdo SF30 Pro, and in Timespinner I can actually configure the type of button prompts to show, I can set them to "nintendo controller" and then they line up with my input device. Using Steam, I can manually reconfigure where A/B/X/Y is on my controller instead of the game insisting that A is the bottom button for example as is the case with base xinput devices (I got 0 clue why Microsoft did this, it's silly).

One day when the Internet dies due to nuclear war, you wont be able to play any games you have on Steam but all GOG games will work just fine !!!