That seems to do the trick!
I modified it slightly to:
[from-internal-custom]
include => from-internal
exten => 2007/2008,1,Log(NOTICE,GDS3710 is calling to extension ${EXTEN})
same => n,GoSub(func-set-sipheader,s,1(Call-Info,<IP_of_GDS3710:443/capture/8001> ;purpose=GDS-view))
IP is prefixed by https:// , I can’t put a link in a post apparently
As this is the information the GDS3710 passes on to FreePBX. I can’t test it right now as I don’t have GXP2140’s here, but the debug and packet capture look promising. I’ll let you know on Friday if it actually works.