The first thing I will say is that I own VVVVVV on the PC, so I'm very familiar with this engine.
The app version of Gravitron works just as well as the PC, since the only two buttons are left and right (I have to wonder how the full game would work in an app). The entire game runs on reaction time and skill, so the controls are smooth and easy to understand, and the rest is up to the player.
Basically, you are in a thing that makes you fall down, then up, down, up, repeat, and you must dodge objects coming at you from the left and right. Simple controls, simple concept.
But it turns out, the game is super flippin hard. Good luck lasting 10 seconds. For your first 50 plays or so you'll be struggling to last 5. And that just makes it more daunting.
There is a bug sometimes where leaderboards don't update and your new time won't be saved, but you do keep the achievement.
You are also, unfortunately, unable to listen to your own music as you play. Many games will silence their music (but keep their sound) if you're playing music so you can play your own soundtrack, and unfortunately that is not an option here. But the game is still well worth it.