There are so many different types of GPS out there and technology has changed a lot since I got my first GPS in 1999.
Not only have GPS units changed from the basic ones that stored a few waypoints and routes and displayed them on a monochrome screen to current ones that can hold a lot more data, display coloured maps, and even include cameras.
Add to that the fact that computers have changed significantly as well and you realise there is not one answer.
Really old GPS units used serial communications and required a RS-232 port on the computer. You will be lucky to find one of these on a modern computer so a work around for that is required.
Newer units used a USB connection instead of RS-232 but it was still the serial protocol.
Modern units no longer "talk" serial; some claim do but fail miserably when asked to do so (I can testify to that after my eTrex 10 got a firmware upgrade).
The GPS Exchange Format (GPX) file is now the way to exchange data between GPS units and Anquet OMN. In fact GPX files should let you exchange data between any devices/applications that claim to use this format.
To try and make this painless I have produced a video named "GPS in OMN" that you can find on YouTube that takes you through the entire process of configuring OMN (including deciding what type of device you have) and transferring data both ways.
This is one of a number of helpful videos you will find information about on The Anquet Guru Website along with other helpful hints on using Anquet products.