Sarries found themselves 13-3 down during the first half at Vicarage Road before a resurgence saw them clinch a 28-21 triumph.
McCall's team are now in second place in the table, three points behind leaders Northampton Saints.
"We were 13-3 down, and they came out of the blocks pretty well. We showed a bit of resilience, a bit of resolve to get ourselves back into the game. Some of rugby after that period was a little better," McCall told the club's official website.
"I thought we bounced back really well. Gloucester worked unbelievably hard, and I don't think we saw that kind of fight from them last year. It was an unbelievably physical game, so we are very happy to have come away with the win."
Saracens begin their European Champions Cup campaign next Saturday when Clermont Auvergne travel to Allianz Park.