Hopefully the OP, CnicK, will see this. I have a fix. This is the best Portal Gun addon of the 5-6 I see on the workshop right now.

The only problem is the odd error spam when a player is moving through a portal in a multiplayer server;

This is (crudely?) fixed by removing or commenting-out the line (with "--") 213 in lua/autorun/portalmove.lua that reads:

After testing this fix in a multiplayer server for over an hour trying to do everything to break it, (endless falling, closing portals on player/props, cleaning map, spamming, etc) I could not get it to generate a single error on a dedicated multiplayer server.