This depends on what chips they use to implement the RS232.
Most RS232 systems work with the transmitter swinging 0V to +5V which is casually called “TTL level”. Actually classic TTL level is something like +0.2V to +3.5V or some such; since the 1980s everybody has been using 74HC logic which swings 0 to +5.
The above works because the receiver usually has a threshold of +1.5V or so. That is what the standard chips e.g. 14C89 use. Or +2.5V if feeding into a 74HC kind of chip, or into a microcontroller UART (bear in mind the UART input will be upside down so to receive RS232 with it you need an inverter).
There are very few RS232 systems where the receiver has a threshold around 0V and then the transmitter needs to swing -3V to +3V as a minimum. The MAX3232 chips do that, from a +5V supply. A more common transmitter swing is -5 to +5 and the old classic chips (14C88 etc) swing -10V to +10V (with a -12V and +12V supply), and the the classic MAX232A swings -8V to +8V.
So if your transmitter swings 0V to +5V that should work unless they have done a right hack.
Anyway I have level converter for arduino. Point is recognize protocol for it. Transmission dump would be key.
Late to the party, but what happened to this project? Any luck driving the Trio Servo correctly? I consider doing the same thing using a semi-professional/commercial (arduino-) auto pilot.
Oh, and BTW, your GPS module seems to be upside down. Not good for the sensitivity 
