As an endless runner, it's quite simple. Some things I can think of that would keep it more engaging for longer is to have hazards come in bursts. As it is, the only the decision the player has is up or down, and about 50% of the time, not moving is the correct choice. Also, increasing the speed of the gravity flip to go along with faster enemy movement would feel pretty good. Allowing the player to flip gravity midair and having airborne hazards is another idea.
Not sure if it will work well given that you had a mobile implementation, but I would have also liked to see the ability to jump alongside the gravity flip. That would give the player more options to respond to what could be coming ahead if there are hazards on both sides
I liked the sneaky spike. Gave me "I Wanna Be the Guy" vibes