Go on a crash diet and lose 10-15 pounds, then start gaining again. The regain is always faster and more intense than the initial gain, and you'll gain past your previous peak.
This! Short, mini-diets have helped me get through plateaus in the past. I incorporate this into my gaining regimen. My gaining cycles last four weeks, with a one-week break in between. During my break, I eat a low-calorie, low-carb diet. Sure, I'll lose a little weight during the break, but it generally helps me avoid plateaus.
If I hit a plateau that even the one-week break can't break through, then I'll just take a longer break. That does the trick.
I resumed gaining near the end of November, 2021. I weighed 212 pounds at the time. I currently weigh 264 pounds, and I'm still steadily gaining.
I wish you the best of luck in getting past your plateau.