When I first tackled the gib I relied on the " good advice" of someone at the local vintage engine club. This technique involved a lot of buggering around with little wedges and shims which tended to drift up.
So I thought about an easier way and decided to make a very big long wedge with a very shallow angle for maximum power and long enough that I could drive the end from outside the diameter of the flywheel.
So I grabbed a long length of angle iron and cut a long wedge with my angle grinder.
To avoid it riding up while driving I cut it with a bias so that the edge bit at the bottom of the key.
I rocked up at the club with my length of angle iron, tapped it snug on to the gib shoulder and drove it with a small mallet. It worked perfectly. Quick, cheap, easy and no damage at all to the gib key. Avoid the temptation to use a shorter steeper angle. I would suggest making it at least 14" long so it's a nice shallow gradual wedge.
2" wide angle would be about right. That will drive out the gib far enough to get it loose.