Its not only that. Pjanic is the finished product at this point while Gomes is a work in progress, so offcourse Pjanic has better stats at the moment. The couple of times I have seen Gomes play he has been hot and cold, but the huge potential is there for all to see. Beppe has an eye for those kind of players like Dybala and Morata fx, so if he thinks Gomes is worth the huge investment I will trust him 100 %.
Perhaps he is, but year after year he's getting better, so I'm not sure if he reached his zenith yet. Considering his technical abilities, and especially his vision and intelligence, I personally expect him to have his best seasons at 28-31.
We are focusing on his stats this season, but we seem to forget that last year too he was serie A's top assistman. For two years running he's the best assistman in the league and that stat is only getting better through the years.
In the last 4 years he has these stats:
2012/13 4th best rated player in Roma; 2nd best in Roma in created chances
2013/14 Top 7 in serie A in average rating; 8th best in serie A in created chances; Roma's 2nd best rated player after Benatia; Most man of the match awards in Roma; 3rd best passer in Roma
2014/15 Top assistman in serie A with 10 assists; 6th best in serie A in creating chances; Most created chances in Roma; Roma's 2nd best passer; Most man of the match awards in Roma
2015/16 Top assistman in serie A with 12 assists; 2nd best in serie A in creating chances (Hamsik created the most); top 5 in average rating in serie A; Roma's best rated player; Roma's best passer
His development is obvious. Getting better every year, to the point that he's in the elite in the last 2 seasons.