The gravity systems in this case are not pressurized. They just have a water tank in the loft/airing cupboard and the hight of the tank determines the pressure. 0.1 bar for every 1 meter height. You open the faucet and gravity pushes out the water.
Its a nightmare, I used to live in UK and these systems are barely enough for anything really.
In my experience the relatively small selection of the F-droid store is full of these type of games.
Troll patrol
Sudoku
Unciv
Shattered Pixel Dungeon
Pirate Solitare
…just to name a few All full versions, no ads or premuim options. Love F-droid, its a shame the selection is quite small.