The link in #2 doesn't do much to deal with the "Gas-" part of the OP (now 14 years old!). The work that it describes relates to water exclusively, as does the term "plumber" in AE (and I suspect BE from dictionary entries, but I'm not sure). In fact, I was very surprised when my husband called a plumber to connect our gas stove--but that's who did it.

As an answer to the OP: I'd suggest "plumber and gas fitter."

#4 would work in AE as well.

Schreib ich mal in die Übersetzungszeile rein - dann findet sich das in Zukunft leichter.

(#2 war bestimmt Werbung, Fadenausgrabung eines neuen Users.)

... oder wie es bei Brösel heißt : "Gas, Wasser, Sch...." ...

scnr ! :-)