It's a compelling argument, Gronk vs Tony G. I could be swayed either way but generally it comes down to whether you value career longevity stats or playoff wins/Super Bowl rings.
If you break down their regular season stats, they basically each had four seasons in which they just absolutely dominated. Their total yards and TDs are pretty close in those years except for 2017 when Gronk had an incredible 17 TDs. Gronk has the rings but Tony continued to produce as a solid TE1 well into his Atlanta days. And lest we forget Tony G. is second ALL TIME in career receptions - and not just for a TE. Literally Jerry Rice is the only person who caught more passes in NFL history than Tony Gonazalez. That has to count for something.
And it's not my favorite argument to make, but look at the QBs each had to play for:
Gronk - Tom Brady
Tony G - Elvis Grbac, Rich Gannon (pre-MVP), Trent Green, Damon Huard, Brodie Croyle, Tyler Thigpen, Matt Ryan
Those Huard/Croyle/Thigpen years were brutal for what should have been the peak of Gonzo's career.