The best gravel to use is NO GRAVEL (sorry for caps but must be noted and taken seriously) the best substrates to use are; sand, large river rocks or nothing at all.
Sand and river rocks thoroughly rinsed are astheticly pleasing but can be hard to clean(axolotls can be quite messy)
A bare bottomed tank is good for ease of cleaning but on the down side certain plants require planting to grow.
On the plants side of things Java fern is a very good plant if not the best plant for an Axolotl tank, it grows well in the tank condidtions (cold water and low light), and withstands the biting and thrashing that will inevitably happen while your axie goes about its buisness. Marimo moss balls are also a good choice IMO and gives your Axolotol something to push around the tank.
There are already lots of threads dedicated to good plants to use in an axolotl tank, a quick search of the forum will unearth a wealth of knowledge!
Cliff