Quick googling here, and I think I understand the setup. Correct me if wrong, please.
1. The Garmin
device sets up a wifi access point.
2. The PC should connect to that access point using the configured name/password.
3. This gives the PC an ip address to use. I understand that you can see this ip address when connected correctly.
4. So, you should, in theory, be able to create a connection in OpenCPN
using that ip address.
5. We do not know if the data from the GPS device is sent by UDP, TCP, or multicast. This is pretty important. I assume you have tried UDP and TCP?
How on all that so far?