12s Di2 means ultegra and that's only a 32t on the back isn't it. Not a huge range. If you want GRX then it's 11s anyway. You probably want 2x but if not, 11s 1x you can use an XT RD with road shifters
12s road cassettes run up to 34t, and 12s di2 is also 105. Some say you can run 36t cassettes.
I think 36 is the minimum I'd want on a gravel bike but if you use a sub compact double then 34 would probably be alright.
@snoops started
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12s Di2 means ultegra and that's only a 32t on the back isn't it. Not a huge range. If you want GRX then it's 11s anyway.
You probably want 2x but if not, 11s 1x you can use an XT RD with road shifters