I work in the mechanical trades and have a gas tech license.
My advice would be not to waste your time and money pursuing just a gas technician certification because on its own, it isn't very valuable to an employer and is quite restrictive. HVAC is not a trade in itself, more a combination of trades that come together to build more or less one system.
For example, say you are a gas technician. You are allowed to physically install an appliance (put it into place) and do the gas piping and the venting for it. However,
-You can only connect it to existing ductwork, you can't actually install any ductwork except for the one piece that goes directly from the appliance to the existing ductwork (you have to be a Sheet Metal mechanic to install any ductwork)
-If the appliance has a potable water connection to it (like a hot water tank), you can only connect to existing water piping for that appliance, with a limit to how much pipe you can install (I think it's something like 5 ft laterally) (you have to be a Plumber to do any more). Same for a hot water heating system.
-You can run wire to the unit to power it with electricity, but you can't install a circuit breaker in the distribution panel (must be an Electrician)
-You can't touch the air conditioning side of things at all, unless you are a Refrigeration Mechanic
Now, I'm not saying that none of that doesn't happen (mainly residential), but the point is that those are the real HVAC trades (minus electricians, they generally aren't concerned with how a piece of equipment runs or what it's for). There are a lot of people in those trades who also have a gas technician certification basically as an add on to their main trade license. I would suggest trying to get in with one of those trades because contractors don't often pick out a job as a gas job; gas is just part of the rest of it, and companies who just have gas techs aren't really in the same league. Of course this means low pay and fewer opportunities. Service is a bit of a different story, but generally you have experience with new installations before you go on to service.
Don't go to a school like Hi-Mark because they have a reputation for teaching their students how to pass a test and not how to become technicians. If you want to go to a college like Humber or George brown and take a 2 or 3 year course, they are good but more geared towards office stuff like sales than to going into the trade.
Best route would be to phone around the local union halls for those trades and find out when they are taking applications for apprentices. Also phone contractors and see if they are taking apprentices. A lot of places will pay for you to get your gas technician license, especially if you are with a unionized contractor.