My personal favorite are the RealPresence apps from Polycom, available for iOS and Android. They offer you to sign in with a Polycom account, but you can simply skip this and just register with any H.323 gatekeeper in the settings.
Collaborate Mobile is also a good choice and also available for iOS and Android from the respective app stores.
Yealink VC Mobile is relatively new, but seems to work find with GnuGk, too.
There used to be an Android version of BeedHD, but now its only available for iOS.
I have dropped Sony IPELA from the list, because their apps seem to hang up without any visible reason after a few seconds in the call.
Here is my older post about free mobile H.323 endpoints for reference.