When you select a chord set, Scaler 3 will automatically reflect the scale of that chord set. To lock the chord set to a specific scale of your choice, click the lock button next to “semi” in the right section of Section A’s header.
So for your example, the flow would be:
Select G Maj
Lock chord set to current scale
Select Common progressions → 1 1V V 1V → Thickened 2
You can search for Scales. Check out this part of Davide’s in depth Scaler 3 overview, or check out page 29 of the user guide.
Have you checked the Scaler 3 User Guide as Steven wisely mentioned (in my case it’s page 31 though ?
Teaser : it is the padlock icon on the right of the A area.
I confess that when I started my Scaler 3 journey I was searching it in the Chord Scale area (B) as I wanted to lock, well, the Scale. So it’s in A, I’ve integrated it in my workflow now and after a few minutes it’s not so painful anymore