There is no easy way to do networking because networking is not easy. No matter what engine you use you are going to have to mess with ports and sockets, some middle-man server, or both.
I don't know how 'legacy' you are referring to with your version of GameMaker. If it is any version of Studio then it has some decent networking functionality built-in. Make sure you check the manual and you should be able to readily find tutorials by doing a search online. Even the forums have some decent tutorials in the
tutorial section.
If you mean 8.1 or earlier then, if I recall, the networking was pretty defunct due to the tech that was used (I could be wrong as I don't recall when they replaced the old system, I want to say it was w/ Studio). People used things like the 39.dll made by 39ster for decent networking. I think there was one more option but I can't recall the name. I don't know how valid these DLLs would be at this point but you could look them up, I suppose.