Gravel as an aquarium substrate is not a bad idea and I would say the majority of aquarists use it as their bottom. There are things to look out for when choosing gravel though and some invertebrates such as shrimp and snails would probably prefer sand. Some Chichlids would probably prefer sand also as some species like to dig for example.
Gravel can be more difficult to clean as debris can find its way underneath it as well as on top. But plants may benefit from this as the debris is broken down.
Also think about the size of gravel pieces. smaller pieces would allow burrowing invertebrates such as the assassin snail to burrow easier than larger gravel. Also smooth gravel is better for bottom feeders with delicate mouths.
If you buy gravel. Be sure to wash it first. Get the right gravel that bests suits your stocking such as size and shape also different coloured gravel will compliment different coloured fish.