Yan Gomes was one of the biggest surprises in all of baseball last season, as he went from not making the 25-man roster out of spring training to putting up the highest slugging percentage of any player on the team with 300+ plate appearances and playing arguably the best defense of any catcher in the American League.
It was sensible to expect his offense to drop off this year, and sure enough, it has. He's striking out more, his unsustainably high BABIP from 2013 has fallen to roughly league average, he's got a batting average 49 points lower than last year's, and a slugging percentage 66 points lower. I think those figures will rebound a little as the year continues, but his offense is pretty close to what I expected, and I'm fine with that, because he's still an above average hitter with these numbers, and above-average offense combined with elite defense makes one a great player. What has surprised me so far is the big jump in defensive mistakes Gomes has made.
After making only 3 errors in 710 innings behind the plate last year, Gomes has already made 5 errors in 122.1 innings this year, 4 of them on throws and 1 on a dropped pop fly. After allowing only 4 passed balls all of last season, he's already up to 3 of them this year. Those 5 errors and 3 passed balls are each the highest number by any catcher in MLB this season*. He's already almost half way to J.C. Arencibia's league-leading 11 errors from a year ago, and 30% of the way to Arencibia's league-leading 13 passed balls.
*Whatever is a little off with Gomes right now, it was off during spring training too, when he made 3 errors in 104 innings.
Gomes' arm is strong, which does a lot to limit opponents' baserunning, and his pitch framing continues to be good. Factoring in all parts of a catcher's defense, Gomes' overall play has still been very good this year. It would be better though, if he could get a handle on whatever it is that's leading to these miscues. I cannot find monthly splits, so I'm not sure if passed balls and catcher errors are more common early in the year (though with Gomes leading the league in both, even if it's normal to have more trouble early on, Gomes is having more trouble than others).
I don't think Gomes' new contract is an issue (that's a common explanation from many any time a newly signed/extended player struggles with something, but I believe that explanation is nonsense in 99% of all cases), and I don't expect him to have this amount of trouble all season, but he is having trouble right now, and it's something to keep an eye on as the season continues.