How is it uneven? It's okay if it slopes a little from back to front. Some filter plates [the plastic piece for the bottom of the tank] are even taller in the back to accommodate this.
The only real problem would be if there were extreme differences. The water will tend to flow through in the spots where there is less gravel and it might make other areas clogged, or less dissolved oxygen and water will flow through those areas.
A little unevenness won't hurt.
