Loving the progress so far! As always, I have some critiques:
1. For some reason, I keep phasing through the fruits when running as Alex.
2. The title theme sounds alright, but the rest of the music reminds me of the Crazybus soundtrack... and not in a good way.
3. The arcade game and the boss fight are both braindead easy. I actually had to intentionally die during the arcade game because I got bored after playing it for a few minutes. As for the boss fight, you can literally just stand in one spot and jump when the robot approaches you to defeat it.
4. The level design often asks you to jump across small platforms. However, Alex has very slippery controls, especially when running. Slippery controls aren't inherently bad, but it makes the platforming unnecessarily frustrating. Either adjust Alex's controls, or adjust the level design.
5. The level design on display here is very, very basic, not a lot has changed since GoD1. The upward drafts in TT2 are neat, but nothing interesting is really done with them, it's essentially just a special type of platform that only works if you glide. There's barely any challenge, the majority of the levels is just "jump across this small gap, now jump across another small gap, now jump up through a semisolid, now jump down through a semisolid, rinse and repeat". I understand not wanting to have super-complicated levels at the beginning of the game, but there's a difference between "simple" and "boring".
6. The purple fruits don't really seem to have a point. They look too different from the regular fruits for anyone to accidentally pick them up, and avoiding them isn't ever a challenge. They don't even contribute to your total fruit, so I'm not sure what their purpose is.
7. There are a lot of graphical cutoffs. The biggest instances I noticed were the big stone tower decorations, and the hills in the background of TT1 & TT2. The green bits of the hills really don't transition well into the blues underneath them. And of course there are a lot of smaller examples elsewhere in the game. I know that you're dealing with actual GB limitations here, but there are ways to artfully work around them. I'd recommend taking a close look at the best-looking GBC games and seeing how they deal with this stuff. One thing that I definitely know you can do is this: finish the game, however long that takes, and then look at how much space you have left in the virtual GBC cartridge. Then, starting with the most glaring graphical issues, start using up that spare space by making extra tiles to address the cutoffs.
As always, good luck with the game! I'm very excited to see how it turns out!