I doubt the gasget expanded. The tank did.
I had to move a 75 gallon h.w storage tank that was under some stairs so the Carpenter could work on the stairs. I moved it out of the way temporarily and moved it back. After I moved it back and bled out the zone everthing seemed fine but i noticed a small leak coming from the bottom of the tank where the supply from the zone entered. I took off the little panel and the insulation was saturated, the coil cover was rusted and all 8 nuts were corroded, the drip was coming from the gasket between the cover of the coil and the tank. I brought this to the attention of the homeowner and told her that the coil cover needed to be replaced along with the gasket and nuts and that the corrosion around the tank might cause it to let go at some time.
She told me that the tank was 10 years old and that the company that installed it said it would last FOREVER! Well I told her that nothing last forever and that this tank certainly wouldn't and suggested that she call the installing company and have them replace this forever storage tank. She called them and they told her that the tanks probably on the way out and they'll send someone out...................Forever Ha!
Actually after the storage tank reached full temp the gasket expanded and the leak stopped.