During our Top 100 end-of-season prospect discussion over the weekend, readers asked for an updated take and grade on New York Mets rookie right-hander Robert Gsellman. Since coming up to the majors in late August, Gsellman has made six starts and one relief appearance and has been excellent, posting a 2.56 ERA with a 35/14 K/BB in 39 innings. He will retain rookie status for 2017 and you can make a good case to put him on a Top 100 list.
When he arrived in the majors we posted this profile, noting his strong 2016 minor league performance. The most important point was a reported velocity increase: he was at 89-92 MPH in 2016 but was reportedly throwing 91-94 with peaks at 98 this summer. In the majors he's topped out at 96. He's added a cutter. Sabermetrically a key takeaway has been a notable increase in his strikeout rate over the last 12 months, attributable to both the fastball velocity increase and the improvement in his secondaries, without loss of control.
I had Gsellman as a Grade C pre-season and moved that up to C+/B- when he was promoted. After getting a better look at him in the majors I think a Grade B is appropriate, which would put him into contention for the next edition of the Top 100 (which should come out in February).