I also got the stamp for helping those two before I got my DIY (and before I started keeping track of how many times I helped Gulliver). Eventually I did want to know how many times I still needed to help him, and I did find a way:
What I did was look up a list of Gulliver rewards, then searched for all of those in Nook Shopping to see if they were in my catalog or not. If you haven't traded to obtain any of his items, you can simply count how many show up, because there are 29 unique rewards (with the 30th being the DIY).
Side note, it's surprising how little he shows up when you actually want him for the progress towards the DIY, lol.