Gravity XO is a XO board game, it is a variation of a classic XO game, just like games as "4 in a row" and "Drop down XO" are.
Goal is to have 4 same balls in a row (horizontally, vertically or diagonally). Plus, there is a restriction, new balls can be placed only on the free fields that are next to previously placed balls, hence the "gravity" in it's title.
This game was actually my student project on AI (Artificial Intelligence), and now, many years later I decided to refresh it on Android. And Gravity XO is a result :)