That is because, sourcemod base plugin already create first sv_gravity command.
Then this second plugin duplicate same command name.
You will have two sm_gravity commands
*edit
And I don't think
SetEntityGravity use same gravity value as server sv_gravity.
https://sm.alliedmods.net/new-api/en...tEntityGravity
It's percent, Gravity to set (default = 1.0, half = 0.5, double = 2.0).