These suckers are on a neglected ornamental pear tree at the gym I go to. They seem pretty solid like attached to a big stout root.. Can I dig them up and use them for grafting rootstock?
You could. As long as you get some roots with it, they should do fine. The only reason not to do it is that ornamental pears can be invasive. So any suckering or failed grafts that eventually fruit will spread that seed onto your property. Some places there are already lots of wild bradford pear trees already so grafting onto them isn't changing anything.
If you want low cost pear rootstock, I'd suggest taking seeds from pears you buy at the grocery store and growing those. Then any roostock that escapes is still a useful and manageable tree.
If the suckers don't have roots you can place a tube (bottomless tube) over the sucker, fill with soil about 8 inches, and water and let grow roots for a season.