Are you sure it's gas in the oil? Gasoline is a solvent, and generally is invisible in the oil. It just thins it, and makes it stink like gas. The concern with this issue (besides the cause) is that you could cause crank bearing failures due to hot spots and lack of lubrication.
White foamy looking stuff on the oil cap is usually a sign of water in the oil. Plenty of it, usually on the dipstick as well, is a sure indicator of a blown head gasket, cracked block, cracked head, and possibly a blown intake gasket. Anything that leaks water into nearby oil passages. Most often, this is a blown head gasket.
Head gaskets can blow in different ways. Compression from cylinder to cylinder, or cylinder to outside. Water into oil and vice versa. Or compression into water. A compression or leakdown test would find if compression is leaking, but not necessarily if water is leaking into the oil. For that, you would have to pressure test the cooling sytem.
It's also entirely possible, if only a very small amount of water is present in the oil, that its just condensation. I see that in race motors every so often when we pull the valve covers. Surprising amount of white oil, sparcely spread.
An unexplained loss of water over time would be an indicator of a head gasket also.
If it's an EFI engine with an electric fuel pump, then there's only one way that fuel can get into the engine. And that's due to leaky injectors. If one sticks open, it can drip (engine off) or continually spray (engine on) into the combustion chamber. There, it will end up washing oil off the rings and cylinder walls, contributing to premature cylinder wear if not fixed. After rinsing past the rings, it ends up in the oil pan and mixes/dilutes the oil.
Injector O-rings generally only seal the injector to the fuel rail or the intake manifold. O-rings should not cause any flow issues. If O-rings are bad, the injector leaks externally.
To a much lesser degree, a weak ignition (bad plugs, wires, possibly coil) could cause a misfire, and enough misfires will leave unburned fuel in the combustion chamber. Left misfiring, fuel could wash the bores and end up in the oil. But the motor would run like crap, and you would know it.