I'm not an expert, but it seems that gargs (and cresties) grow at relatively slow rates. I have a garg and a crestie I bought recently who are about the same age. The garg was about 18 grams back in April when it was 7 months old. It now weighs 40 grams. The crestie, which I was given because it has a severe underbite, was 10 grams when I got it in July at 10 months. It's now 18 grams. If your garg isn't eating crickets, it may gain more weight if you add those a few times a week. Sometimes the garg needs to be held and a cricket pushed into its mouth so it finds out how good they are.
Aliza