If there have been diesel spills on the gravel you won't be able to grow anything in the actual ground unless you can clear the gravel and any polluted soil beneath it. You can, however, use containers of varying sizes and shapes filled with plants and small shrubs to make the area attractive.

It sounds like your desire to establish a front garden means that the gravel layer removal is a good possibility. If you do remove the gravel and if contaminated, dispose of it in a proper manner I would suggest that you not let an assumption hinder you. Proceed to plant your garden as normal and see what happens. I do say though you should have made it a stipulation that the contamination be cleaned up before you purchased the house ( I am assuming that you are not renting). Should you have a basement, fumes from the contaminant could enter there and permeate throughout the whole house and that could be a very bad health hazard to you let alone the plants. Did you get a home inspector to look over the property before you acquired it?

Very few homes in the UK have a basement Loosestrife and they tend to be larger older houses where there would be no need to cover the front garden in gravel to park vehicles on it.

As a thought Dimwit if you do remove the gravel you may well find a weed suppressant membrane underneath which may have helped to protect the soil. You might also want to contact your local Council and ask their advice on how to dispose of the gravel as it is contaminated. If there isn't too much you could bag it up and take to your local recycling centre to process.

It depends how much diesel was spilt. I've been driving diesel cars for 30 years and never spilt any of the stuff so not sure why you would think it has been spilt - unless the previous owners told you that they had.

Probably meant oil from a leaking crankcase.

Loads of weeds grow abundantly on my gravel drive, which has had plenty of diesel dripped on it over the years; surely it would take a really big spill to do any real damage.

Are they not making properly sealed car gas tanks anymore or are people gassing their cars up with cans of fuel right in their driveway?( blackmarket style)

Compost tea often helps with minor oil contamination, but may take years to help with anything major. I would still grow only annual flowers on the area for several years, and bag and bin the dead plants in fall, to help remove any long-lived contamination from oil additives.