Top soil! [emoji41] u won't be disappointed. Remove all the gravel put in a layer of top soil 1 inch thick (just plain organic top soil with no added ferts), then top it with your gravel. This is the cheapest option.
There are many planted substrates on the market these days so u can use any of them. Most people use, ADA, brightwell, Tropica, Eco complete, fluval stratum, flourite.
It's really personal preference as to what look your after.
U don't need a planted substrate to grow low light beginner plants but if u want to go into higher light and pressurised co2 then it's a must.
What plants are u planning on keeping?