Toilet gasket not holding water. Need a temporary fix.

You could try brake silicone grease or plumber's silicone grease (Home Depot or Lowe's); I had a leaky toilet ball seal and cleaned it as Old-Biscuit described. Then smeared silicone grease all over the sealing surface and so far it has held water.

When I cleaned the gasket, I found that it was very rough, from the build up of something. After I cleaned it, it was smoother.