Oil can leak from a head gasket, yes. Though to be honest, it's usually a 'weep' or a drip - not a big oil leak as you describe. Depending where there is a tear/damage in a head gasket, a car can either leak coolant to atmosphere, oil to atmosphere or oil-water mixed inside the head/gasket. The latter is called a 'blown head/gasket'. You obviously don't have that.
It's difficult to see why a bump causes an oil leak. The only thing that crosses my mind is that the sump has been nudged and is no longer seating properly/damaged. Are you absolutely positive that you had no oil leak BEFORE the bump?
Other than that, an oil leak is usually caused by a gasket/seal failure.
Can you clean the engine and get underneath with a torch whilst the engine is ticking over? You need to identify where the leak is coming from before you can do anything about it. Just a note of caution .. don't drive the front wheels up diy ramps and let the engine tick over. Because of the angle of the engine the oil pump won't pick up oil from the sump and things will seize. I speak from experience..
Remember that oil drips down .. I'm not making a joke .. but .. you're looking for the highest point on the engine where you can identify the leak. It can be a messy business indeed.
If push comes to shove, take your car to a workshop (or call a mobile mechanic) and ask them to identify the cause of the oil leak. Sometimes it's worth paying a few quid for a diagnosis rather than stumbling around in the dark. Once you have a diagnosis you can decide whether you do the job or let somebody else do it.
Get under the car with the engine running/torch and try and find the source of the leak. It's frustrating and annoying, I know .. but just remember that we all go through this sort of problem. I have a 1994 900cc Cinquecento that is a hypochondriac. 30,000 miles from new and I've had to replace most things. Tomorrow I'll need to remove the head because of a knocking/rattle.
Don't worry unduly - you'll suss things out eventually.
