Gems in Forsaken World come from several locations. You can reliably find gems in some instances such as God's Trial and as drops from dungeons. Additionally, you'll get a few gems for free by simply leveling up your character and opening your supply stash every five levels. You can use these gems at the blacksmith in Freedom harbor. Firstly, though, you'll need sockets. Sockets open on your gear once every three levels of fortification. So, at a fortification level of nine, you would have three open sockets. The maximum number of sockets any item can have is four.
Once you have enough sockets for your gems, you can begin socketing them in. Each piece of gear can only take specific gem types, so pay attention to what each gem says it can be socketed into on its tooltip. Additionally, only one gem of each type can be socketed into an item. This means you won't be able to stack up on physical damage alone - you'll have to delve into other bonuses.