You have just built or inherited a pond and you want to put gravel in the pond for the effect, is it a good idea?
Makes the pond look nice, changes the contrast so the fish stand out, relatively cheap to do.
Need a lot of gravel to cover the pond floor, can puncture the liner.
Now some facts:
Yes gravel is cheap, yes it can give good contrast, yes you do need a lot of it, but yes it can quite easily puncture a flexible liner, but even if its a "solid" liner gravel is not a good idea in ponds.
If you look after your pond you should clean it out once a year. When you do, what do you find? sludge, decaying matter and other debris, now where does that go if you have gravel? it sits in and around the gravel that's where. So instead of a relatively strait forward pond clean, you now have to carefully remove a whole load of gravel, you can't use a shovel for fear of damaging the liner, so you have to use a plastic dustpan (Good luck) Then when all the gravel is out you have to wash it, then put it all back.
You can get a pond "under gravel filter" it has no UV, and is not that popular, but these you have to have a 10cm layer of gravel (more hard work in the first place) and you still have to clean the gravel, yes not as often, but there is also very much more of it.
You can use a pond vacuum to clean the gravel? no you can't all a pond vacuum does is remove the larger dirt particles, it does not wash the gravel.
Still want gravel in your pond?
There is more to a pond, how it works, how to look after it. Please look out for other articles for your guidance.
Good quality costs. Bear this in mind before you start.
Sorry if my reply is NOT want you want to hear, but what I have said is true.