You shouldn't be using plugins from the old, deprecated pgb repo. Use the latest versions of the plugins from npm, instead. Be aware that they are named differently (e.g. cordova-plugin-inappbrowser).
Also, the inappbrowser plugin now has a different API (functions for the plugin are named differently), so you need a couple of code changes).
Thanks, this put me on a good direction but still getting the error for iOS. Works fine for Android.
<gap:plugin name="phonegap-plugin-push" source="npm">
<param name="SENDER_ID" value="XXXXXXXXX" />
I'm not seeing anything specific in the documentation for iOS installation.
Edit: Even manually re-installed the plugin via:
phonegap plugin add phonegap-plugin-push --variable SENDER_ID="XXXXXXX"
Same error for iOS.