OK go to Control Panel>Device Manager. Look under following headers to see the COM/serial ports allocated below COM6:
Modems - look in device itself and click on Modem tab to see COM port allocated.
Mice & and other pointing devices
Ports (COM & LPT)
Look specifically for any COM port that has yellow exclamation mark by port icon. This means drivers have not been successfully installed. If you find such or indeed any others you don't need, right click on each & select uninstall to free up.
Typically COM1 will be Windows pre-allocated generic port. COM3 is usual modem port. Don't touch these.
Note that your Garmin USB device may appear with port allocation under Ports (COM & LPT). If this is occupying a lower port number try double clicking on it, then select Port Settings>Advanced. Here you will be able to change the port number to something higher & free up the original. Note you may then have to re-configure input to GpsGate.
Any COM port allocation you have free'd up should then be available for virtual port creation with GpsGate unless Windows has messed up.