What would cause gas in the oil of a 22 hp brigs and stratton engine?

One reason would be that the float valve may be sticking in the carburetor and letting gas flow by and into the engine when the mower is not running.

The other thing I have seen cause this is one cylinder is not firing and that cylinder lets the gas by the rings and into the crankcase.

Most of the time it is the first thing I listed.

You can change the oil and install a shut off valve in the gas line. After you mow just shut off the gas valve. That will let you know if that is the problem. If it takes care of it you can either have it fixed or just use it with the shut off.