I've just finished building a small pond using a liner, but I'm not sure if
it's a good idea to put gravel in the bottom or not. Being a tropical fish
keeper, it seems the natural thing to do, and I think it will look better,
but will it be a pain to keep clean?
Any thoughts much appreciated.

We did rocks and things in the veggie filter and we let plants just
grow wild (and I go in and yank bunches out whenever it gets ugly),
but the main koi pond is all black liner with two bottom drains.

I highly recommend bottom drains if you want big Koi BTW...