download ZDL Zdl.vectec.net and then put it in the same folder as gzdoom. run ZDL, click settings, and then set your source port and IWADS.(it's very straight forward and you should be able to figure that out yourself). After you've set all those, go back to Main, click on the IWAD you're going to use, and then click on Add under External Files and then add all the wads you're loading on top of doom(brutal doom, a hud mod, ETC). Once they're all in there, set the load order of each one by clicking on any listing in external files and then using the two arrows next to Add and Rem(remove).
Once you've set everything up, click the arrow that i've circled in this picture.
Prntscr.com If you're planning on joining a server, click Game Mode and then change it from single player to Co-op and change Players to Joining. If you're hosting one, then still set Game Mode to Co-op, but this time also set the player number to whatever number of people are going to connect.
For this last part you can use a variety of different services such as Hamachi or EVOLVE, but we'll be using LAN bridger for this tutorial.
download Lanbridger.com and then install the appropriate version. If you're hosting a network, then click "Host a network", click New, set the name of your server and the password, and then hit okay. Next, click the first icon from the right of where the name of your server is displayed(it should look like a radio/cellphone tower). Give the link it gives you to everyone you want to be able to join your server. All you have to do now is go back to ZDL and click launch.If you're joining the server, then just copy the IP it lists for the host and then paste into the spot I've circled in ZDL. Prntscr.com then hit launch and you're done!