Apro - we've got "guru"s here that aren't really (I'm one) and plenty of other people like Ernie that don't appear to be one yet from their post count, but really might be. So IMO, don't put too much stock in the titles.
I've got zero experience with gravel only, but just because you have fish in the tank doesn't mean you can't add substrate. Eco-complete is expensive, but it pours beautifully underwater. Adding it won't bother your fish at all. Maybe some other substrates pour underwater too. Just keep in mind that if you do add substrate, after it mixes up a bit with the gravel, the gravel will make its way to the top, burying the substrate, which might be just what you want.
Steve - 33g reef and a 180g planted in need of a re-scape.