I plan to run 700C with 38mm tires (maybe 35 or 40 also but never less than 35mm) and 650B with 2.1 tires.
24mm for the 700C and 35-40mm tires seems to be fine indeed.
The GR are good. The GRC are more for the aero advantage IMHO.
I originally was looking at their CX wheels but now with their gravel specific wheels with 24mm this is a better option.
For the 650B with 2.1" tires, that's where I am not sure at that point which route to go.
DT Swiss recommends 25mm to 33mm:https://dycteyr72g97f.cloudfront.net/uploads/WXMC120AEIXCA05040/MAN_WXD10000000866S_WEB_ZZ_001.pdf
Their gravel wheels have 24mm.
Their XC wheels in 27.5" have 22.5 or 25mm.
However their lightest XRC are only 22.5mm.
For their All mountain the inner width increased with 25mm to even 40mm!
However their lightest wheels XMC are 30mm.
So basically if you want the lightest 650B wheels it is either 24mm, 22.5mm or 30mm.
The XRC are the lightest (-230g vs. the GRC) but the GRC have the aero advantage (which may not be that useful for XC profiles)
Okay, I just think the recommendations of DT Swiss are a bit extreme, and not really the industry standard. But if you think weight is really important, it will always be very expensive carbon wheels above 2k USD. If money no object and somehow you favor (in my opinion unnecessary light rims) then this would be the choice: https://www.enve.com/en/products/g27/
Internal width of 27. I think 2.1 tyres would fit well on it, but not so sure about Resolutes of 42mm wide. To cover all bases perfectly, I still think 25mm internal is the way to go. https://www.enve.com/en/products/m525/
This is a mountain bike wheelset with 25mm internal. DT Swiss does have wheelsets for MTB with 25mm internal as well. Personally I would just choose the midlevel newly released GR DT Swiss series with 24mm internal, that 1mm difference won't be an issue at all, and if the max you want to ride is 2.1inch it's even slightly better than 25mm, as if may fit 42mm 650b better.