Gruber was one of the founding members of the Mattachine Society, a homophile organization. The papers contain letters, clipped news items, draft writings and photographs.
![[Mattachine Society Christmas Party in Los Angeles]](https://digitalsf.org/sites/default/files/styles/enhanced_grid_small/public/externals/693fbdb6b436faeb3d4ec6a078453a66.jpg?itok=AX-ydnil&pid=islandora:154828&iic=true)
