you should add a Photon Multiplayer plugin. Here is a site for the multiplayer engine [https://www.photonengine.com/]
Checking the docs Photon appears to only support c++ and c#. The sdk would not be compatible with GDevelop, so unfortunately a plugin for that wouldn’t be feasible.
However, if it has a web enabled REST API you can use the network events to make calls to their system. That still wouldnt be a plugin or a extension, but you’d be able to use it through the event system.