I have a GMNT-080-4B Some times I get a 3 blink light issue and then when it starts it has a flame roll back. Also I have to change the ignitor every year. Any help would be great

Flame rollback can be from a various number of issues all being dangerous. Best advice would be to call a qualified technician so he can check gas output levels from the valve, heat exchanger, etc.

I just checked my install manual online, whould a under size vent pipe cause the flame roll back, the installer put a 2" and should be 3"

Is it 2" all the way to the outside or does it go from 2" and then transision into 3"?

It is 2" all the way to the roof, I went to the roof to see if it was clogged and saw that the venting pipes was 2"

Well then yes that is incorrect. The vent size standard off that furnace is 3"... sometimes a tech will downsize at the furnace for approximately 3 or 4 feet and then go back to 3". Being that it was installed with 2" it causes a retriction for proper combustion flow. That unit is designed to flow with a 3" pipe and therefor should be installed as such. I would call them back and tell them to fix their screw up. Are they paid in full?

Yes it was installed a few years back and paid for, I just kept calling them and now they won't call back. So I was checking to see if I could fix my problem instead of calling another company

You could replace the pipe yourself with 3"... make sure you use schedule 40 DWV PVC pipe though. Send the contractor a certified letter stating he has 10 days to respond to you in writing his intentions to fix his improper installation. A lawyer would probably handle this for you and collect his fees from the contractor. I don't normally suggest things like that but I also don't like to see someone get taken advantage of. Just a thought...