I do hope you're going to dig the existing plants up and give them away to people who will enjoy them!

Do you really want gravel instead of a garden? Once you HAVE given the plants away, you'll need to lay a membrane before getting the gravel in. It will need to be a thick layer, and you'll have to watch for odd seedlings and pull them out before they get too big.

If you offer the plants on your local 'freecycle' they will be collected by someone who can perhaps make use of them? But ideally its the wrong time of year for transplanting.
Gravel is not problem free animals may use it and it may shift and weeds do grow on top in the debri that collects in it?
Clear the ground well, level it and compact it, You need edging to contain the gravel, I used railway sleepers for 'impact', I added 2 layers of membrane before adding a larger size pebble ( this was my shaded narrow side garden) and then I planted Junipers in it.