A ContaxG is a basic standard electronic camera just like most others. It disassembles the same, using the same screwdriver as any other camera. Sticky switches can sometimes be cleaned up using contact cleaner just like any other camera. It seems that too many believe that every camera is some complex unique piece of technology. If you don't feel comfortable trying to service it yourself, just take it to any competent camera repair technician.
FWIW, I have had both ContaxG and Zeiss Ikon camera bodies repaired by technicians who had never seen one before mine.