Replace rotten bottom plate
If the bottom plate and studs were installed to finish the garage, only to support the drywall, then replacing the bottom plate would be fairly easy. You'd have to remove the drywall, remove the nails from the studs into the bottom plate and them hammer out the bottom plate. If there's any weight from the top plate, you might need a ceiling screw jack to temporarily support it. If any of the studs are rotting, you'll need to cut out the bad sections and sister in some replacement sections. Be sure to use PT wood. You'll also want to find out where the water's leaking in from the outside and seal that off.