Show all group content of a specific group type
Wow, I actually figured this out. Basically you need to do the following:


Add the "OG Membership: OG membership from Node" relationship to the view. 
Add the "OG Membership: Group Node from OG membership" relationship, assigning the above relationship.
Add a Filter Criteria of "Content: Type" = the group content type you want to show, and (here is the trick) assign the above relationship for this. 


Here is a screenshot that might be more helpful: