OK, so for what it's worth, I have another phone that has Android 4.2 and Gizmo plays my videos just fine on it (at least on my LAN). I suppose I should put my sim card in the 4.2 phone and see if it will play outside my network.... I bet it will.
So it appears the problem is either with the Nexus 5 device, or Android 6. Can anyone confirm video playback on Gizmo with Android 6.... or 5 for that matter. I suppose I could roll my phone back to 5... although I'd rather not mess around with rooting and all that.
I really like the potential of having one app that gives my phone access to audio, images and video from my home server (with some flexibility to organize and search). Messing around with uploading to Google music for audio, and some other cloud service (Onedrive, Tonido, etc) for images and video hasn't worked.
Thanks in advance.