I've been using the 100gr GMX in my 6.8SPC AR on whitetails for 5 or 6 years now and have had good results with them. My gun likes them (and it's not a high dollar AR build. Rock River barrel on a built upper & lower). Shoots 1MOA consistently. Terminal performance has been good and always get clean pass throughs with good blood trails. I don't think anything has gone further than 75 yards or so.
I like them on a small caliber like I'm shooting because I feel like I want to pick up any extra penetration I can. I'm putting together a .308 for next season and I'm going to try out some solid copper loads (TTSX & GMX) but I'm also going to try some lead jacketed stuff like the Berger, Accubond, SST etc. bc I don't feel like I necessarily need all the penetration available in the bigger caliber especially on whitetails inside of 300 yards which is my hunting situation.
My $.02.