I have been working with polyurethane often and I get some on my hands but mostly my brush. I try to wash it out right after I am done using it but it does not seem to work, even with simple soap. What can I get that will clean this fun stuff up? Could I use gasoline (it works with paint). I hope this is not off topic!
As with any finish you need to use the appropriate solvent.
For waterbased poly you only need to use water (warm water ideally) although soap and water will generally work much better to get a brush really clean.
But from the wording of the Question it sounds like you're using oil-based poly, so use mineral spirits or paint thinner.
Could I use gasoline (it works with paint).
Old-timey guides often suggest using gasoline as a general cleaner but modern gasoline is not the same as the stuff of old and many modern sources suggest not using it for this sort of thing any longer.
If you need a cheap cleanup solvent paint thinner is usually cheaper (sometimes much cheaper) than mineral spirits, although it also tends to have a much stronger smell that some people find objectionable. It's also marginally more toxic, although if you use good ventilation this shouldn't be a concern.
It's also much cheaper in the long run to buy any solvent in bulk than in smaller quantities, so buy in gallons, not in quarts if possible.
Remember that whatever solvent you use to clean your brushes it can be recycled multiple times, at best almost indefinitely: How do I properly care for my paintbrushes?