A warning to all: audio does not work full-duplex when calling from a VPoint (VCON Emblaze) H323 client -> GNUGK -> Asterisk-oh323 -> SIP client. Tested with versions 8 and 10.

However audio works perfectly in the other direction.

The solution would appear to be to use OOH323. However the audio is again half duplex in the scenario where SIP clients are calling H323 clients.
Therefore calls originating from Asterisk to H323 clients use the ‘OH323’ module, and calls from GNUGK into Asterisk use the ‘OOH323’ module.