Bumgarner (8-5) tamed the Rockies on four hits, issuing three walks while striking out five in posting his fourth victory in five starts.
Pence's two-run shot in the fifth inning broke open a close game and handed the Giants a 4-0 lead.
Drew Pomeranz (0-1), called up from Class AAA Colorado Springs before the game, was touched for four runs on seven hits over 4 1/3 innings for the second-place Rockies, who remained two games behind National League West leader Arizona.
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