Hey GOGers,
I have a great idea for the Galaxy client. I enjoy watching Let's Play Youtube videos of my favorite games. Other players help me learn secrets and techniques about the games. I also enjoy watching a really skilled player play games that are hard to beat and they do it easily. Even though many of the videos nowadays are recorded on HD, they are still blurry compared to actual game play from the game. That got me thinking about how Galaxy could capitalize on this. Imagine if Galaxy had a function where you could record your keyboard and mouse input for a game and upload it to share with other players. They could download the input file and have it play the game for them from Galaxy. This would have many advantages over Let's Play videos. First, the image quality is perfect because it's running directly from the game. Second, gamers with slower internet can still watch the game in HD quality. Of course there will be pause/save and resume function, so you don't have to watch the whole playthrough in one sitting. The main reason I want GOG to add this function is safety. It will be a built in Galaxy function on the client, so people can't hack into someone's keyboard and mouse. I think this would be a wonderful addition to GOG and many players would use and enjoy it.

You might want to create a wishlist for your proposal:
https://www.gog.com/wishlist/galaxy

I hate to spoil your dream, but that's not really possible. Old games like Doom and Quake had an in-game recording function, and it was indeed extremely cool to playback things like Quake Done Quick through them in-game.
For for that to work the position and actions of every single object have to be recorded. It doesn't work just by recording inputs and playing them back. What about AI behaviour, dice rolls, etc?

And even if a game was totally deterministic (same input -> same behaviour), it would only work for games that not dependent on timing in any way. Even animations playing a little faster/slower would break the sync and thus the playback.