How long has the G3 model been out. How reliable is it? what is the company support like or do they dump your over to the local place where you purchased the unit?
Portage & Main has been around for many years about ~40 off the top of my head, they own Polar Furnace. The G-class series has been sold since 2011.
The g3 and g2plus owners I spoke to were very happy and didn't have any complaints and are impressed with the efficiency and ease of use. One boiler is 6 year old and the other is 4 years old with no wear points that stuck out.
After looking at one in person I knew they are built like a tank and meant to last. The firebox being 304SS also sold me. No worries about corrosion failure.
Like a previous member stated they are based off a Froling design which is also another proven reliable boiler.
If your installing one indoors you will want a boiler with good smoke extraction. Nothing worse than stinking up a garage with smoke. That I don't have a problem with anymore after I upgraded to a Polar.