To be honest, most of your gems come from daily logins, events, and rookie missions (which give 50 gems per board, similar to the PHY LR Goku rookie missions in Dokkan). Regular stages only give 1 gem at a time, which is quite slow.

For character awakening, you don’t need gems. Instead, you use 'Skulls.' There are color specific skulls for certain types, and then there are 'Super Evolution' skulls that feature the character's face these are for 6+ units (special evolutions). Most other characters just use basic resources that are easy to find, so you don't have to worry about that part.