How to make a gamerule that if true it spawns mobs.

on the mobs spawn condition I suppose

yeah, spawning conditions

or do you mean that it spawns literally besides the player when game rule true?

Yes.......... Put closely attention: On the Spawning section of the mob click the + button on the bottom where it says Spawning Condition

Then put this

if logic game rule "GAME_RULE" = true

 do return true

return false

indeed yes