Dirty sock syndrome refers to a condition in which the indoor coil in the air conditioning system/heat pump develops a musty smell similar to that put off by dirty socks that have been sitting in the bottom of a laundry pile for a week. The exact cause of this syndrome is unknown, but once the problem rears its ugly head (foot) it's almost impossible to get rid of it.
All brands of air conditioning systems/heat pumps can get it. It's more prevalent in coastal and high humidity areas, and it usually shows up when the seasons change. The problem seems to be more common in systems where the air filters are not replaced or cleaned or properly maintained. Oversized equipment appears to contribute, so if the equipment is old enough that you are going to replace it, make sure the new equipment is properly sized.
The first thing we would do is have you run the blower in the "auto" mode. This helps to reduce the humidity inside the house and reduces the chance. Clean the indoor coil thoroughly. We would spray the drain pan, and indoor cabinet insulation. This works about 10%-20% of the cases, but it's certainly the least expensive cure available. If the system is still under manufacturer's warranty, the manufacturer will require you to attempt this procedure before they'll offer any type of other repair/service. The next possible solution is installing an anti-bacterial ultra violet light next to the coil. Different brands of UV lights may yield different results, depending on the strength and wave length of the light they emit. Ask you Gossett technician what would be the best for your home and/or situation. The last ditch effort and most expensive is to replace the indoor coil.