The biggest knock against JG was that he had showed very little in both the preseason and in his periodic appearances. But I don’t recall him being considered a bad practice player. Belichick usually complimented on his role running the scout team.
Wait... what? This isn't true either.
In 2014 he got into 6 games, going 19-27 for 182 and 1 TD (101.2 rating). He played well in the KC game, then was pretty meh in the season finale against the Bills (when the Pats were resting starters).
But he was also very good in the preseason in 2014:
Week 1: 9-13, 157, 1 TD
Week 2: 6-12, 72, 2 TD
Week 3: 9-12, 105, 1 TD
Week 4: 22-42, 284, 1 TD, 1 INT
Total: 46-79, 618, 5 TD, 1 INT, 99.0 rating
I remember being wowed in the very first game against Washington.
Edit: here's the 2015 preseason
Week 1: 20-30, 159, 1 INT
Week 2: 28-33, 269, 1 TD, 1 INT
Week 3: 13-17, 126, 1 TD
Total: 61-80, 554, 2 TD, 2 INT, 92.4 rating
2016:
Week 1: 11-18, 168
Week 2: 16-21, 181, 1 TD
Week 3: 9-15, 57
Total: 36-54, 406, 1 TD, 95.1 rating
Total preseason:
143-213 (67.1%), 1,578 yards (7.41 YPA), 8 TD (3.76%), 3 INT (1.41%), 95.5 rating