In real world terms there is about the same amount of work in putting a route plan into a GA GPS as there is in putting a route of the same time duration into an airliner FMC.
The FMC will of course put in all the waypoints on an airway a with just the input of the start fix, airway number and end fix but most of this information is surplus to requiements in practical terms.
The time that most likely to need a "surplus" waypoint is if ATC wants an ETA for that point, the nearest function on most GA GPS units is likely to find that point and get it into the flight plan as least as quickly as it can be done from the fix page of an FMC. As to loading data in lat/long format there is nothing to choose between a GPS & an FMC in terms of time.
There are some navigation functions on the FMC that the GPS lacks but add a heading input from the HSI and a Fuel/Air data computor and you get 98% of the functions of an FMC for 5% of the price.