There can be several reasons for gas in the oil.
Could be carb issue. Engine starts running rich, all of the fuel isnt burned and it ends up in the oil. Another carb issue could be a leaking needle or needle seay. Allowing gas to get into the cylinder after shutdown and leak past the rings into the oil.
The most likely way is what is called "short tripping". Short trips here and there, never really getting the engine up to operating temp for any period of time. All internal combustion engines will get gas in the oil. But running them at temp for a while will allow the fuel to evaporate out. Also over time, plugs get old and dont fire as good as they once did. Theres some more unburnt gas getting into the oil.
If you notice it being way over full on the dipstick, its likely a carb problem. A slow increase in oil level or just the smell of gas in oil, I would vote for short tripping.
On a closing note, make sure that thermostat is operating properly. One that is stuck open will never let the engine fully warm up. If you havent ever changed it I would do so, along with a coolant change.