In lieu of using gasoline or other solvent that will need to be discarded, many freewheels just need some lube and not any kind of flushing to work perfectly.

I keep a squeeze bottle with motor oil mixed with 20% solvent that I use as a general lube and penetrant prior to servicing old weathered bikes.
It goes on all of the threaded fasteners, pivots, derailer pulleys and metal-to-metal cable runs. It can even be used for temporary lubing of dried-out bearings on lesser-quality bicycles (sufficient to get the bike on the road for evaluation so that servicing over a period of actual use can be facilitated).

For freewheels that don't have seals, I put 20 drops into the drive-side gap between the inner and outer freewheel body, this for freewheels that are presumed to be virtually "dry" inside. I don't then normally have to re-visit the freewheel innards for some years, I save time, and am left with zero petro products to dispose of.

Cleaning a crusty freewheel exterior (and chainrings), I leave it dry and start with mechanical removal of inter-cog detritus using a stainless steel flat ruler against the sides of the spinning cogs. Depending on how gummy the deposits, the detritus may or may not simply fall off onto the grass at the side of my house where likely no one will step on it and track it around.
For tougher or "gloppy" deposits on bike components, gasoline is not a very aggressive solvent at all versus the petro-based Finish Line Citrus Degreaser, which aggressively dissolves almost all detritus except for water-proof grease (water-proof grease is troublesome if it is in a bearing retainer, almost impossible to clean out!).

Moral of the story is to use as little solvent as possible, and to keep all degreasing products away from your skin and from the air that you breath.