The response you get from using garlic or supplements is going to vary from horse to horse, bug to bug, and area to area. If you have a good management program already, and your horse isn’t overly bothered by bugs, I think using either is a bit of a waste IMO.
However, they do work for some horses who have problems with bugs, have severe reactions from bites, etc. But again, the effectiveness depends on the animal and the environment.
Back to the OP, as far as I’m aware, small doses of garlic will be okay, but you may want to check out other substances.