Polycom
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.
http://support.polycom.com/PolycomService/support/us/support/video/realpresence_mobile/realpresence_mobile.html
ClearOne
Collaborate Mobile is also a good choice and also available for iOS and Android from the respective app stores.
https://play.google.com/store/apps/details?id=com.clearone.collaborate
https://itunes.apple.com/us/app/collaborate-mobile/id824952324?mt=8
Yealink
Yealink VC Mobile is relatively new, but seems to work find with GnuGk, too.
https://play.google.com/store/apps/details?id=com.yealink.videophone
https://itunes.apple.com/de/app/yealink-vcm/id1114669846?mt=8
Radvision
There used to be an Android version of BeedHD, but now its only available for iOS.
https://itunes.apple.com/de/app/beehd/id639912661?mt=8
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.