To answer your question in a nutshell: IF you are using NMEA 0813 the answer is -- no, it is a straight output to input. This assumes you are just connecting two devices. When you add a 3rd device, you'll still be OK (using a terminal strip to join them) so long as it is only a "listener" and you don't have more than one talker. If you have two talkers they will step on each other. If you have multiple talkers you need to either use an NMEA multiplexor, or convert to NMEA 2000 or other "bus" architecture.

This issue of NMEA in and out is age-old in RS-232 serial port communications. Also known as data in and data out. The confusion comes from the reference point: Is the reference point for "in" the NMEA signal source, or the device displaying it? Your manuals should give you a hint, but you need to read both carefully. Hook them up the way you think is proper, and if they're not communicating at first look at the data rates in the settings on both devices, expressed as "BPS" or "baud" rate. They must match. The other settings might be number or bits per character, use of parity, and stop bits (convention is 8, n, 1). Your Garmin CP will have settings you can access through the menus, and your VHF might too (or it might be set to one specific rate that must be matched by the GPS).

With regard to signal polarity the good news is you only need to test with two wires out of 3. Ground is ground, so you can connect the grounds or "minus" (usually black) and then you can connect the other pairs (in-out, out-in) with test leads (short wires with clips on both ends). If that doesn't work, leave the grounds connected and try (in-in, out-out). When you have it working with test leads you can make your permanent connections. (Don't test by holding with your fingers, because you aren't steady enough and your skin is electrically inductive.)