garmin handheld gps to arduino?

I have a garmin 64st which should be able to send NMEA data.
Which a rally trip computer to build in mind, I would like to connect it to a arduino (or maybe a stm32 board) and process it's gps data.

Unfortunately no one can tell me how I can get the serial data out of the garmin.
Does anyone has an idea how I can connect the garmin to a microcontroller?

I've found a usb to serial adapter? Could that be a solution and how can I test if it works so I know for sure when I don't get data in the controller, the gps exports it?

Garmin will be happy to sell you an outrageously overpriced serial data & power cable:

If all you want is speed and coordinates, it is much cheaper to buy an Arduino compatible GPS module, than that cable. The module would probably work about as well as the Garmin.

There used to be small companies selling much cheaper cables for Garmin GPS units, so spend some time with Google.