I do not have a great deal of info about the shop and artists. I do believe the ones signed GM are first name Glenda, from the signature on a few pieces in the same handwriting style. From seeing all the pieces together, I believe GM could be a relative of JT Mulroy, possibly Glenda Mulroy, a talented artist in her own right from the look of the work. I believe Bill was an early partner or artist at the shop in it's earlier period and that it is a Bill Hanson from the later Black Dragon Tattoo Studio in Minneapolis. RH is the signature of an artist at the shop in the middle or later period, possibly (probably) H standing for Hanson, a son or relative of Bill Hanson. Same goes for the signature TH, which is a similar style of signature to the RH signature. Other signed pieces were likely swapped for with other shops or collected, or artists who trained at the shop at one time or another.
Feel free to ask me any questions you need to know and I will definitely hit you back with my best answer. Good luck, and enjoy!