I'm working on my first game as well. I am making a basic platformer and using youtube ALOT to show me how to program in gml. I'd say don't worry about learning EVERYTHING that the tutorial teaches you. The more you do it, the more it will naturally stick.
The 3 biggest things that I've picked up with learning gm/gml are:
1) DON'T COPY & PASTE: Its fine to use the same exact code if you're learning, but if you dont type it you wont remember how or what you did. This also applies when you are using multiple lines of code that may be very similar to one another (ie. doing vertical & horizontal collisions). Of course copying and pasting, then editing the code, would be more time efficient, but if you're looking to learn....don't copy/paste it.
2) Comments/Notes will be your best friend. Seriously, do it even with simple self explanatory syntax like "room_restart();" Just throwing in a simple comment explaining what you're doing, will further ingrain into your head what is happening, and how the logic/syntax works. And if you are going at a slow pace, or have forgotten what you were working on before, leaving many comments are super helpful for jogging your memory.
3) Use what you have: If you're good at writing but are just learning gml, make the mechanics really basic for a run in the mill platformer, and let the writing be the shinning factor of your game.
I'll end it with this: Don't get too ambitious, make something you can finish and be proud of. Even if the game sucks (which honestly, both of ours might) you can be proud that you finished a project and learned some new things for the next one.