I agree with Fairbanks56...the cable is likely to be some variety of RG-58. However, I would strongly suggest that you find a way to mount the GPS antenna with NEW cable -- RG8X or other low loss cable, because:
1. You don't know the condition of the existing loran cable; it could very well be deteriorated through age, water
intrusion (water wicks along the strands internally), corrosion
of inner braid), etc.; and
2. RG-58 cable has high losses at the frequencies used by GPS; RG-58X or the more exotic low-loss cables are better.
GPS is a pretty important system to have aboard and presumably you want it operating at full capacity. That means it's got to receive a strong, clear signal from the antenna.
There's gotta be a way to put in new cable...how hard can that be on a 27-footer? :-)