I have the Gunson Gastester. I won't claim it's a "high quality" tool. It's actually somewhat cheaply made and delicate but it seems to work.
Yes, power up the Gastester and let it stabilize for about 5-10 minutes. Then adjust the trim to read 2%CO. It'll drift with time but I don't bother tweaking it.
Here's the Gastester in action. I'll be breaking it out again as my 999S is due for another 6000mi interval service and I'm about to fire up my Monster project.
"Far better it is to dare mighty things, to win glorious triumphs, even though checkered by failure, than to take rank with those poor spirits who neither enjoy much nor suffer much, because they live in the gray twilight that knows not victory nor defeat"