My first shop was gravel for 2 years until I could afford to concrete it. I totally understand this situation.
I have been in a couple buildings that had course lime for flooring. Course lime is pea sized and smaller crushed limestone waste. It packs like cement. But inside a building it dries out and turns to talcum powder.......
Probably stick with your gravel until you can swing the cost of concrete. Try to anticipate your grade level for concrete and make that the top of your gravel. Then whenever you can afford concrete the existing floor will be "close" to grade and require minimal work for the transition.