Connecting Garmin 18x 5hz GPS to PC via Serial TTL-232 USB cable
The Garmin uses a somewhat bastardized version of RS232 that is designed to work with most PC serial ports. It is using RS232 signalling but without a proper driver IC so the signal voltages are 0-5V rather than the full +/- range RS232 should use.

The cable you have is for connection to a TTL (logic) level UART. You need to add a line driver to get the correct signal levels to connect to a normal RS232 port.

In other words they are both using 5V compatible signalling but the Garmin is connected as if it had a driver, the FTDI is connected as if it needs one. The driver ICs not only boost the voltage, they also invert the signals.
Your data lines are 0 when they need to be 1 and 1 when they need to be 0.

The simplest solution would be to use a normal RS232-USB converter rather than a TTL level one. Failing that you need to invert the signal levels or add a line driver to the FTDI cable.