The Premiership champions were almost going home without even a losing bonus point as the game entered the final moments.
They were 17-9 down after Gloucester full-back Olly Morgan had scored a super try, and fly-half Freddie Burns and his replacement Tim Taylor contributed some kicks.
But ex-England international Charlie Hodgson and centre Owen Farrell kept Saracens in touch before, at 17-12, Farrell's grubber kick to the posts saw Brad Barritt just get in ahead of the covering Dave Lewis for a score-levelling try.
It still needed a touchline conversion from Farrell to grab the win, which he did and completed a great personal display.
However, McCall wants more from his team.
He said: "We were poor and I can understand why a lot of teams come down here and get beaten because of the intensity.
"The fact that we were able to stay tight and not lose the run of ourselves, that we showed the composure in the last 20 minutes said a lot about us.
"Owen goes from strength to strength. We moved him from the centre to number 10 and he took over the goal-kicking duties from Charlie and kicked well.
"But overall, as a team performance, we have to do a lot better. We have got a run of big games. It was great to win on our away game but we talk about performance all the time and today was not one of our best."
Gloucester head coach Bryan Redpath claimed his side threw away the game rather than lost it.
The former Scotland scrum-half said: "There were three or four dull penalties in the last 20 minutes.
"You can't give away cheap penalties in your half, which we did. Credit to them, they went to win the game (in the second half) as they had nothing to lose.
"They had a grubber kick at the end and scored but I think we were in control and there were not that many chances they had of scoring a try."
England international Mike Tindall came on in the second half for his first game since the World Cup.
Redpath added: "He is training and playing for Gloucester well and, if he doesn't train and play well, he puts himself in danger of not being selected, the same as everybody else.
"The key thing is we have got him back playing and he can enjoy the game of rugby again."
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