Just posting this here to help anyone who googles this in the future.
If trying to use the plugins.findPlugin function in Gizmo, be aware that it is specifically looking for the plugin signature, which can be found alongside the Loaded: line in X-Planes log.txt
So if you are looking to check whether plugin admin is loaded (bad example, its always loaded unless you've broken something) then look for the following line in the log:
Loaded: X:\X-Plane 11 Beta/Resources/plugins/PluginAdmin/64/win.xpl (xpsdk.examples.pluginadmin).
Then in your lua script use the following to check for its presence:
if( plugins.findPlugin( "xpsdk.examples.pluginadmin" ) )then
--we could find the plugin and are capable of sending messages to it in a useful manner.
end
The Gizmo API suggests looking for "Win.xpl", but this is not what Gizmo is looking for. (The API actually states plugins.findPlugin( signature ) but the example shows .xpl)
Cheers.