Some people use lava rock chunks (not gravel) for things like Vandas because they don't really need any soil at all they're just used to hold the plant in place. At Orchids Ltd some of their display Vandas just hang from a wire suspended from the rafters with their roots trailing from above my head to the ground (over 6 feet) but this is a greenhouse that is almost saturated. I'm usually wet as if I'd been standing in a sauna by the time I leave my visits to OL. Most other orchids would do best in orchid bark holding moisture and nutrients at the roots long enough for them to absorb it. Most gravel is smooth and water simply runs/right through with no retention. You can certainly try it but use an expendable plant and see how it goes before redoing your whole collection. You should definately repot the Phalaenopsis if it has peat in the soil. Avoid using peat on any orchids except terrestrials such as Cypripediums, Aplectum, Goodyera, etc. Common tropical orchids need very high air exchange for their roots. Often large market orchids will be grown in peat to bring them into flower faster so they can be up for sale faster but this is often at the expense of the plant who really can not cope with the wrong sort of soil for too long.
For Phalaenopsis, Phragmepedium, Paphiopedilum, Oncidum, Cattleya, and other large rooted epiphytes I use medium grade orchid bark or a mix of medium and fine grade orchid bark.
For things like Miltinopsis, Miltonia, masdevallia, pleurothallis (larger species), equitant Onicidums and other small rooted orchids I use fine grade orchid bark. For some like Dracula and many other Pleurothallids I'll use LFS & fine bark. About 75% bark 25% finely milled spahgnum. Grind it up with your hands before you wet it, this way you get tiny bits that will blend easily with the bark also when you have to unpot them and remove the soil at repotting time you won't destroy too many roots this soil mix should really just fall apart.
Now I can tell you that I've been expeimenting with using cedar mulch for potting my plants instead of the more expensive orchid bark as I use bark in all my soil mixes and it gets very expensive when you have a ton of plants but since I've only begun using it in the past month or two I can't say how the plants like it over the long haul but so far so good. If all works well it'll be a huge savings for all of us.