Do a bit of research on Robert Johnson. A lot, maybe the majority of his songs were open, eg, Walking blues. Clapton's Unplugged version of that is also in G.

A lot of electric slide is in G, but since I mostly use open D, I can't recall which ones. Have a look at Elmore James.
Johnson often played Open A. He would tune to open G and put a capo on the second fret.  
My Suggestion is Rolling and Tumbling, Come and Go Blues, and Crossroads Blues.
"Music became a healer for me. And I learned to listen with all my being. I found that it could wipe away all the emotions of fear and confusion relating to my family." Eric Clapton