IMO, gem tangs are the easiest of the large tangs. While yes there are some relatively easy Acanthurus tangs (Olivasceus, Tennetti, Pyroferus/Chronixis/Tristis ect..) I think the easiest of the Zebrasomas are the Gemmatum, Flavescens and Xanthurum. be warned, in a matter of time these fish can reach close to 1 foot (Often staying around 8-9” but can reach 10”+). These fish aren’t as bad of an ich magnet as other tangs of different genera (Acanthurus is the worst for ich). I have never owned a personal gem tang but have kept several gems alive in the LFS I worked in. They are very easy compared to the price tag, if fed algae (I find these guys are best when it comes to Hair Algae and Nori) and frozen they’ll do just fine. I have never had one of these fish go on hunger strikes (I think I kept 5 alive for 4 months and they’re still thriving in other peoples fish tanks to this day) unlike the Achilles where there was always Atleast one that went on a hunger strike but then went back to eating within the next week.
Your gem looks VERY healthy, I would try to get some hair algae growing so that it can always graze in a more natural way (My C. binotatus has short hair algae on the back wall of my tank to constantly graze on and he grazes on it all day, I do this with both tanks and, all of my algae grazers love it - Magnificent Foxface, Twinspot Bristletooth Tang and a Lineatus Blenny). If you want to mix Zebrasoma tangs I would get the gem in first then do another two at the same time. I hope this helped and I hope you and your tang have a lovely time!!