As the doctors are told, "first do no harm". I think part of the confusion is that you are trying to deal with YnGlitch (technical jargon allegedly spoken in English, usually by folks with no skills in either) and there is mass confusion over what is or isn't an 'antenna'.
There are many "hockey puck" GPSes, which a little plastic lump about the size of a hockey puck or a mouse is actually a complete GPS unit, including the antenna and the circuitry to send data out on an NMEA or other line.
But these look pretty much identical to "active antennas", which are usually a mushroom-shaped GPS Antenna --and only an antenna-- connected by a coaxial cable to the rest of a GPS unit. A lot of people have no idea which unit is which, and you really have to check the printed spec sheet for the model number to be CERTAIN.
If the Raymarine radar has a provision for a GPS *antenna* I would be very surprised, since antennas tend to be matched to specific model GPSes for electrical signal, voltage, power levels. The odds are that the Raymarine has a provision for "GPS input" meaning an NMEA-level input form a GPS unit, and not from a GPS antenna.
"NMEA" is very similar to "RS-232" which was the standard serial connection for computer printers, mice, GPSes, etc. for many years. For sailors, they are almost identical and often are treated as identical.
Assuming your GPS unit has an "NMEA out" or "serial out" or "RS-232 out" port on it, those are all very similatr and any one of them can be connected to the "GPS in" port on the Raymarine. (Which might also say "NMEA in" or "GPS output" depending on who wrote the manual. An "input" and an "output" often confusingly are the same, just depending on which end of the same connection you are referring to.)
So, check your printed specs. If the Raymarine takes an NMEA signal from the GPS, you want to connect the NMEA output from the GPS, to the input on the Raymarine. That's an easy wire run. If your GPS already connects to an autopilot--that's probably the same wire and yes, it usually is OK to just make a "T" in the line and connect the one wire to all three devices.
Cutting into a GPS antenna cable, on the other hand, is not such a good idea. Coaxial cables need special fittings and tools, and the simple act of cutting the cable changes how it will carry signals "forever", no matter how well it is spliced or repaired. That can degrade performance, and if the Raymarine really has a connection for a GPS antenna--I'd be surprised, but then I'd say to install a second antenna, right on top of the radome and just for it, redundantly.