Main goal of the game is to collect as many as possible red gems. Kill other players that will bother you with bombs. Keep an eye on health level to restore it with aid kits in time. You can use minimap to define your current location and gems' locations in the game. The white dot is you, green are for aid kits and blue ones show the respawn points of your opponents.
Press Space button to place a bomb in Gembrawls io. You can have only 3 bombs placed at the same time. As soon as bomb explodes it's back available for placing. If you press Space for a bit longer you'll throw the bomb towards you. Be sure to release a bomb before counter is down and bomb explodes in your hands.
When item (gem or aid kit) is picked up it will re-appear on the same place after a while.