I used to have that same kind of snail (it was about the same size too). It was also named Gary after Spongebob's pet snail (I didn't name it). The thing sadly died though. About how long have you had your snail.
when my dad had his old tank, i picked one out and he bought it for me, im pretty sure we had to put extra food for that thing lol we got it real cheap like under a dollar but that was like 8 years ago.
The really big apple snails that grow to baseball size are Canas. The come in gold and brown/black. Look for them on Aquabid, there are quite a few on there now because it's pond season. You can get them fully grown from some people.
I love Canas because they are so HUGE! They grow pretty fast, too. They will eat ANYTHING. One mowed down all the plants in a 55 gallon planted I had overnight! They are not recomended for planted tanks for that reason, but I didn't know that then.