Hiya henryb, welcome to MFK!
Now when you say 1/2 water and one 1/2 gravel is the gravel secured i place by anything?
Is the tank divided back to front, or side to side? Any way one way to do this is to place a food grade plastic boin the tank as a stand for your gravel. Holes can be cut in the sides of the box for water flow and as cave for you fish and gravel can be silicone glued to exposed sides to disguise them. glue a lip of pebbles on the lid to retain your gravel so that they barely rise above the water level. I would put small plants potted in shallow containers and snugged down into the gravel. An inch or two of gravel on the tank bottom, an 10" high box, and an inch or two of sloping gravel on top of that would giv you some usable depth and still leave some space for plants while giving you around 12-14 gallons of usable water.