I've got 3 garters that were wc as babies. I started them all out on feeder guppies / mosquito fish (I've read that feeding goldfish exclusively leads to vitamin deficiencies). What kind of garter do you have? At 8" it could handle day old pinkies if it's "into them".
I have found that the western terrestials I have are much more willing to take rodents. My western terrestrial from Humbolt, CA needed his first pinki scented / rubbed on a treefrog, but took his second without scenting. My western terrestrial from Santa Barbara Co. Ca just wanted his brained and I didn't need to scent at all; he was only about 7" when he took his first pinki.
My T. sirtalis from Santa Barbara Co. has probably eaten over 10 pinkis now, but still refuses to eat them unless they are scented. I also had to wait until he was about a foot and a half before he'd even consider going through the trouble of eating even a scented pinki. Luckily he loves fish.
If you do decide to try pinkis, since he is small you'll want to pre-kill them since he'd probably have a lot of trouble overwhelming a live one.
-Alice