Thanks @Xydocq for the thoughtful reply.
In my case, the GX50 is behind the ISP-provided router, which is set up in "bridge mode" (essentially a DMZ, with all router services disabled, including any firewalls).
The error I get on the client side is an authentication error, so it appears the client I'm testing with is reaching the GX50. The problem I'm having is clearly on Meraki's end- we are unable to slide the switch called "VPN access" to the "on" position for any defined user.
Support has said this is a known issue to be addressed in a future update (to both the Iphone app, and the web app, I guess). And just for clarity, we've set up several MX64's, Z3's etc for clients, with both client and s2s vpns, so we are pretty comfortable with troubleshooting this stuff.
What I don't understand is- if I'm using the latest Iphone app (downloaded a few days ago) and the web app (which others must be using), how is client vpn working for anyone?