This list should be complete, but it will not be alphabetically or chronologically ordered. There were not really so many different ones, so it should not be so hard to find your model. OEM production is very common and large companies often have a policy of own-branding all products used in quantity by them.
For Neotronics in the USA there were two main models produced: There was the PCO2500, which was the GA-20 in the ABS case with oxygen and carbon monoxide sensors. The PCO2500 was sold throughout the USA and is not different in any way to the GA-20 of the time. There were cosmetic changes made and software to display temperatures in °F etc, but a PCO2500 can easily be serviced or replaced on the basis of the GA-20. The PCO3500 was identical, except that it was fitted with the optional nitric oxide sensor as well. There were probably more PCO3500 units sold than PCO2500, but there are no relevant figures available from Neotronics at the moment. Both PCO2500 and PCO3500 can be serviced by madur dealers. If the software is updated for any reason, then it is possible that the instrument no longer shows PCO3500 but GA-20.
Neotronics also had a version of the GA-40plus called the PCO4000 with 4 sensors or PCO5000 as 5 gas analyser. Both of these sold quite well, so they are to be seen reasonably often. The PCO4000 was usually O2, CO, NO, NO2, and the PCO5000 the same but with the addition of SO2. Both PCO4000 and PCO5000 used the 3 series sensors. As with the PCO2500 and PCO3500, these instruments can be serviced by any madur dealer. If replacement of the software is necessary, then the instrument may suddenly call itself GA-40plus instead of Neotronics PCO4000 or PCO5000!
For Land Combustion there was a version of the old GA-20 produced, sold under the name Lancom2001. This instrument is just the old GA-20 in the wooden box with a different keyboard saying Lancom 2001. The software gives the same message of course. There were very few Lancom2001 instruments sold, so it is unlikely that one will turn up. They must all be about 7 years old now (2004). The Lancom2001 was also fitted with the silica gel dryer, which should be removed if the instruments are serviced. Land combustion have since developed their own small instrument, so there was no further call for the Lancom 2001.
Since it is a very old instrument, some parts may no longer be available for the Lancom2001, but it should be possible to exchange the electronics for the later version if necessary. The Lancom 2001 used the 3 series sensors, and was fitted with O2, CO and NO in all cases.
The Sensonic 2500 is basically an old GA-20 with the addition of an integrated power supply. The Sensonic 2500 is still produced in small numbers for certain customers. Again, any madur dealer can also repair the Sensonic 2500. The instruments are fitted with the 3 series sensors.