A great vantage point for the GGB is from Marin Headlands. If you are traveling south on highway 101, marin headlands is the last exit on the right before getting to the bridge. You can go up to various lookouts although the lookouts farther up the road close at sunset (don't worry, an officer drives through to remind people before closing it down). As for a picture of the SF skyline, Treasure Island is a great place. There is a large parking lot which has a great view of the SF skyline. I've included two maps and a few pictures from the two locations
Maps
 
Pictures
From Marin Headlands
  
From Treasure Island
  
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