How to insulate around kitchen cabinet cut outs for outlet and propane line access?
The ideal installation would have ducting below the cabinet base to bring the forced air out the front rather than pressurizing the entire space below the cabinet base. I expect very few contractors actually do this. You wouldn't have noticed the leakage going up around into and through the adjacent cabinets; these cutouts only highlight the problem. Depending upon your position as far as contract, warranty, or other leverage you may have with the builder, you might or might not be able to get them to correct this. It would be intensive work, though -- requiring some demolition of the floor of the one cabinet, or else disassembly of the countertop to allow removal of the cabinet unit.

The most realistic/likely resolution is that you make the best of the situation on your own with minimally invasive work. The simplest methods won't look like high-class craftsmanship -- fortunately it's hidden in a cabinet.

You could seal around the outlet by pressing in some foam -- backer rod, for example. Or inject spray foam (I'd suggest the low-expansion door and window type here). The vertical cut-out for the gas pipe could be sealed to the wall in the same way.

The hole through the floor is more tricky. An escutcheon from the plumbing department can help dress up the area immediately around the pipe but it won't be large enough to cover the entire area. It may be easiest to fashion a piece of sheet metal into a two-part cover to fit around the pipe. Small pieces of sheet metal can be found in a home center as duct work in the HVAC aisle or mending/splicing plates in the hardware/building department.