I managed to use my older USB/Bluetooth Delorme Earthmate BT-20 on the FT991 by channeling it through a raspberry pi. I noticed that when I did a cat command on the GPS's usb, it would output the raw NMEA data. So all I did was get a usb to serial adapter, plugged it between the rPi and the FT991, and piped the data from one usb to the other. I used the rPi for fldigi and digital stuff anyway, so it was just an extra cable. Worked like a charm. I was able to get location and time on the FT991 with a USB GPS.