I guess I should explain why I thought oven cleaner would work. I spent 40 years of my life as an automotive and aircraft painter. We used to use Easy Off oven cleaner, to remove paint from some plastic bumpers for repair. It was easier than sanding the paint off. The oven cleaner softened the paint right up and we just used a putty knife to scrape it off. Now, you had to use it quickly because it would sometimes make the plastic swell, but generally it worked really well.
We also used it on aluminum wheels to remove the clear coat. If a wheel was scraped up really bad, we'd strip the clear off to repair it. Again, it was easier than sanding the wheel by hand.
So this is why I thought it would work well on the grip of the gun. I thought it would soften the clear up so I could use a plastic scraper and get it all off. Then once it was dry, sand them and refinish them with oil instead of clear.