If you can do FJ, I think you can do Gringotts. No worries.
Probably not necessarily. I could potentially see a situation were a person's upper body would fit FJ but their lower body not fit Gringotts but that's probably rare.
Here's a photo I found online (not mine).
DSCN0915_zps0e4a76d5 by
suegee57, on Flickr
Funny, we rode it 4-5 times in December, and I could not have told you the ankle restraint was there. I only noticed the waist restraint. So at least to us (I'm not small, and my DH and I both have bad knees), it was very unobtrusive and not at all restrictive. I don't think you'll have a problem.
You must not have sit in row 3. Row 3 has more room but you can see from the picture and I can attest from my trip last week, you're riding with your heals on that ledge and toes on the floor because of the shin guard.
Did you ask for a certain seat or did they just direct you? Im curious about our first trip coming up in a few weeks....Were you also able to ride Gringotts?
On Gringotts, they assign you a seat. That being said, I think they keep an eye open and place larger guests where they are more likely to fit. Multiple trips on Gringotts and ROTM, I got the back row (the one with more leg room) every time but 1. The one time I didn't, ROTM was a walk on and as they were loading there was an empty row 3 they led me to. As it turns out, I fit fine there as well. For people interested here's a picture of me
This next one is on Forbidden Journey. I just barely fit. On the trip around I ended up getting pulled off and a nice ride OP spent 3-4 minutes with me at the test seat working on how to get the restraint done. For me, being both big and tall, it involved sitting down, getting the first 2 clicks on my own, then sliding down in the seat because my shoulders were what was causing the worst problem. At that point, a simple push by the ride up and the 3rd click went in. When I was sitting up straight, it was a smaller ride OP but it just wouldn't go.
Finally, a picture of my on ROTM (unfortunately there are no ride pictures on Gringotts). I had no trouble with either ride getting the restraint done without assistance. But, Gringotts, test seat outside, was about a 1/2 an inch from turning green on my own. (I used the other test seat as a gauge. I sat in the one test seat and pulled the other one down until it just turned green.)
By the way, for those who are curious, these are the sized photos from Universal's Photo Connect. I didn't resize or do anything to them.