This April brings several shows that highlight the power of the ineffable, the pedestrian, and the overlooked. A 60-year retrospective of pioneering LA print publisher Gemini G.E.L. and a 500-year survey of the Gruenwald Center at the Hammer Museum showcase the breadth and depth of works on paper. At ArtCenter, Dave Muller underscores the importance of social connection and the handmade in his oeuvre. Two LA performance art icons — Rachel Rosenthal and King Moody, and Bob & Bob — are celebrated in shows that gather performance documentation with physical ephemera and artworks.…