AFAIK the only way to load an externally generated route into any IFR (panel mounted) GPS, and taking the common GA models only, is if you are using Jepp Flitestar, and then you use the Jepp Flight Plan Migrator program to convert a Flitestar "routepack" into some form which gets written to the Garmin flash card.
This works with a GNS430W, 530W, or the G1000,
AFAIK.
So, no direct computer-GPS connection; it is done via the flash card. It's not exactly cheap, either, on top of the cost of Flitestar.
For a busy IFR pilot it may be worth doing because some IFR routings consist of 20-40 waypoints and entering them with the knobs takes for ever - even if your GPS supports named airways.
The reason for this restriction has never been made clear, but it appears to be concerned with the obvious difficulty of ensuring data integrity when it is being generated by some program running under Windoze or a similar bloated bug-ridden O/S. Whereas if you work it via the narrow flash card route, you merely need to demonstrate (to the FAA etc) that the software which parses the ex-Flitestar routepack file and writes to the flash card, has been developed to certain "software quality" requirements. It has taken best part of 20 years to get this far...