Hi kurb, welcome to Outdoorking.
The first step is to check the oil level. The GXV140 has a one-piece plastic dipstick with cross-hatching in the area where you have to read it, so it should not be too difficult. Remember, Honda dipsticks are read after inserting the dipstick but leaving it unscrewed from the crankcase and just resting against the top of the thread. This is opposite to Briggs & Stratton dipsticks, which like car dipsticks are read after fully inserting and seating the dipstick.
To be able to read the stick, you need to place the mower on level ground, then unscrew and remove it, and wipe the end of it clean and dry. Then place the stick back in the hole without screwing it in, and immediately lift it out again. Keep it vertical so oil will not run up the stick and make you think the level is higher than it is. Use a flashlight if necessary to inspect the position the oil has reached on the stick. It should be right at the top of the cross-hatching. If the stick is wet with oil above the cross-hatching, it is over-filled with oil, and this must be corrected as the first step.
If the oil leak continues when you are certain the oil level is correct, there are several possible causes. In addition to the two your repairer suggested (top oil seal blown out, or leaky head gasket) there are two others: leaky valve cover gasket, or stuck Positive Crankcase Ventilation valve. We can work through all of them, but other than the gasket leaks, they involve partly dismantling the engine. The blown seal or stuck PCV valve both require use of a 6" gear puller to remove the flywheel.
Please check the oil level carefully, and report what you find - then we can move forward a step at a time. In my opinion it is better to do this by investigating the cause before attempting the cure. Also, where the oil is coming from is not necessarily the key issue: if it is overfilled with oil, or the PCV valve is stuck, oil is likely to come out somewhere, regardless of the quality of the gaskets.
