Unfortunately, Gulmite drivers and sockets are super hard to find. We have 3 Crown buses and I've only been able to get lucky to buy one sized socket. For all the other sizes, I use two different "screw removal" pliers. One is a pair of Vamplier screw removal pliers and the other is an Engineer brand. I ground the end of the Engineer pliers, to make the tip smaller for better access into tight spaces. If the larger Vampliers fit your spaces, though, they're super sturdy pliers and work well for lots of things besides Gulmite screws.
I just posted photos of both brands of pliers in another thread here, just last week. Here's a pic of the ones I ground down: