I also only have a GX20 from Cisco. WiFi is provided by a Ubquiti AmpliFi HD running in bridge mode.
The first time I set it up, I connected to it using.a cable from my PC, because I had to change the settings for the ISP which can be found on the Configure tab on http://wired.meraki.com. This was the set up:
Second, and subsequent times (because I had to reset the GX20), I found that just having the GX20 on the existing network, the Meraki Go app finds it just fine, although I still need a web browser to change some ISP settings and prepare it as the gateway device to the internet. First, like this: