I'm not sure how taking Gronk has an effect on whether or not you choose RBs or WRs in later picks. In almost all leagues, you're starting some combination of 5 RB/WRs and drafting 10ish, and the order that you draft them in is pretty irrelevant unless you skew heavily in one direction or the other. So going Gronk > RB > WR > WR > RB > WR is not all that different from going Gronk > WR > WR > WR > RB > RB since they're all gonna end up in your starting lineup anyway. The important question is not which positions you're taking, but which dudes.
Taking Gronk has a greater effect on your QB choices. Most teams will choose their five starting RB/WRs in the first 5 rounds. If you take Gronk, it means one of your other starters has to come from the 6th round instead. If you take Gronk and an early or mid QB, one of your starters comes from the 7th round or later. For a 12-team PPR league, it's the difference between starting the likes of Michael Floyd, Matt Jones, and Jeremy Lankford (5th rounders) vs. Tyler Lockett, Michael Crabtree, and Melvin Gordon (7th rounders). So, if you're taking Gronk in the first, it's probably worth it to go late at QB.
All of this is kinda irrelevant if you pick the right guys. Taking Gronk early should be a function of how confident you are at finding starting-quality talent in the 7th+ round. If you're spending the second half of your draft getting dudes with high upside but not great early chances for production (rookies, handcuffs, etc.), then Gronk is not a good choice. If, however, you spend your second half picks on value dudes you feel confident you can plug in to your lineup in the first few weeks, then Gronk looks more attractive.
Personally, I like taking Gronk in the first, somewhere in front of Brandon Marshall and Jamal Charles but behind Lamar Miller and AJ Green. But if I do that, then I know that I want to look for people like Willie Sneed, Frank Gore, Marvin Jones, or Kamar Aiken in the later rounds. These are guys who I can get late who can go into my starting lineup in weeks 1-4 and not put up a total dud.