The list of the 38 occupations for skilled workers is here:
http://www.cic.gc.ca/english/immigrate/skilled/apply-who-instructions.asp#list Plumbers and steam fitters are both on the list but gas fitters are not.
For occupations under pressure in different provinces, means it's easier to get a work permit if you decide to come in on a temporary work permit and apply for immigration later, for AB plumbers and steam fitters are also both on the list, for BC they have steam fitters and gas fitters. You can see those lists at
http://www.hrsdc.gc.ca/eng/workplaceskills/foreign_workers/occunderpres.shtmlSo from that, I would say that general plumber is better because you have the option of FSW. If you would rather be a gas fitter or it works out better for you, you would have to find a job online and come in on a temporary work permit. You can look for jobs in Canada at
http://www.jobbank.gc.ca/srch_eng.aspx?ProvId=99&Keyword=&OfferpPage=50 to see if you can find more plumbing or gas fitter jobs and which pays better.
http://www.workdestinations.org/occupation_keyword_search.jsp;jsessionid=42223B0771CC8DC385159916B5D65B82?lang=en will tell you what you have to do to get licensed in different provinces. I have heard that plumbers in AB have to take a test within 6 months of starting to work and if they don't pass, they go home. I don't know about gas fitters and don't know about other provinces.