and the gaster blasters where when one shows up you get 3 seconds before they shoot a laser

A cooldown?

Typically when you describe something properly you can actually get the solution for it.

So for showing the blaster and then shooting 3 seconds later. In that case you’re dealing with time, so use a timer set to 30 (3 seconds) and then shoot the blast.
To only be damage by something while moving, when you move turn a switch on to be damaged, then turn it off when you stop moving.
For the gravity part, you can’t use Flowlab built in one. Instead use a Motor pushing the Player down. Only activate this when you want gravity enabled, then turn it off when the player is no longer affect by gravity.

I only just learned what Sans is. I can help if you describe how the various objects work int he original game.

cough Bad Time Simulator (Sans Fight)