Google Voice Gateway will authenticate to your Asterisk PBX if challenged. Follow this example to set up GVGW as a friend and match incoming calls by auth. username rather than by peer IP.

  

[general]
; ... various settings
allowguest=no
match_auth_username=yes
register=GV18005551212:password@GV18005551212/EXTENSION

; this section must be named for your GVGW username in order for matching to work
[GV18005551212]
type=friend
username=GV18005551212
secret=password
host=gvgw.simonics.com
qualify=yes
context=your_context_for_incoming_calls