The toxicity is measured in mammals. We don't have any values for reptiles yet as far as I know. Garlic contains Alliin, which changes enzymatical into some other substances. Those are known to cause haemolytic anemia by destroying erythrocytes (they denature haemoglobin). But this effect is seen only after eating a really big amount of fresh garlic (e.g. 5 g per kg body weight in dogs, which means a 30 kg dog would have to eat 150 g garlic daily for a week or more). First signs for an intoxication are vomiting, breathing too fast, bloody urine.
I don't think we can compare our reptiles to the situation in mammals easily. And we just have no studies done whether garlic's toxic in reptiles (and if so in which species) or not. I don't think a little piece of garlic given once a month to the feeders (not the reptile) will harm anyone.