To put it very simply, BCG Gamma does a lot of the coding and data analytics work that is required during a client engagement. Correspondingly, it prefers candidates with more computer science, programming and data analytics background (rather than people with business background). BCG Technology, on the other hand, is more similiar to traditional management consulting, with a focus on technology and digital transformation. These two departments very often work together during projects, with the former focusing on the technical side while the latter focusing on the business side.
You can find more details on what exactly they do/have done online in their websites. Hope this helps.