The Kittle and Tonyan comparisons don't really line up for me. Both those guys are quite a bit smaller and both were under utilized in their respective programs/schools (imagine having Kittle in college and in 25 games he only had 48 receptions?).
For me when I look at Kmet I see some Gronk. At this time he's far from being that good as a blocker, but the strides he's made suggest it's possible. Remember, that was a large knock on him in the draft. From a height/weight/speed perspective they are very close:
Gronk
Height: 78.25 inches
Weight: 258 pounds
BMI: 30.34
Arm Length: 34.25 inches
Hand Size: 10.75 inches
Wingspan: (N/A) inches
40 Yard Dash: 4.68 seconds
40 Yard (MPH): 17.48 (MPH)
Kmet
Height: 77.6 inches
Weight: 262 pounds
BMI: 31.33
Arm Length: 33.00 inches
Hand Size: 10.50 inches
40 Yard Dash: 4.70 seconds
40 Yard (MPH): 17.41 (MPH)
The both win in similar ways. They can settle down in zone coverage and have a big catch radius. They aren't running away from anyone, but when they have the ball in their hands they are a 260 lb man in a secondary that's a lot smaller.
I'd add, Gronk was only targetted over 100 times four times in his career. He's not a high volume guy, but rather an efficient, splash producer who was a red zone hog (86 TDs in 131 games is just stupid for a TE). In 871 targets he got 86 TDs. He scored a TE on 10% of his TARGETS. Just to compare, Randy Moss had 156 TDs with 1741 targets.