DAX,

Every IF has to have an ENDIF
code:
IF @IPADDRESS0 = " 10. 1. *. *"
USE m: "\\gbtsrv01\users"
ENDIF

If @IPADDRESS0 = " 10. 2. *. *"
USE n: "\\gbtsrv02\users"
ENDIF
Suggestion, you want to clean up the IP addressess using the following FAQ and specifically - IP Address Trimming.

HTH,

Kent
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Utilize these resources:
UDFs (Full List)
KiXtart FAQ & How to's