Sram GX which length cage for 11-46 cassette?

Short - when called X9 or whatever - would do 11-36, i.e. 25 tooth chain capacity. Go long if you are unsure that medium will have the chain capacity - you need 35 which seems a lot. It’s probably in the tech docs somewhere.

I don’t know if the geo will allow a 46 without some kind of adapter though. Chain capacity is independant of max sprocket size, which seems to be 36.

I’m assuming you are talking about this

https://www.sram.com/sram/mountain/products/gx-2x10-rear-derailleur