Do you have to have a plumber to hook up a gas appliance? I see people do it themselves but I don't think they are supposed to.?

The problem is that smart people who know what building codes are and work up to them just because it is right and safe also know that they are smart people who understand the codes, work, and tools.

The ignorant people who DON'T know those things often don't know that they are ignorant people or how much they don't know. Which is a problem.

people who do it themselves do hook it up to the building standards and are qualified to install  it , arent they !!!

Not really the plumbers job. He deals with water.  You need the Gas man.  Usually they do it for free...because they have the GAS LICENSE.  A plumber could be the handyman.(no license required)  Fire insurance companies DEMAND IT OF YOU.

Do *I* have to have a plumber?  No. My state allows any owner of a single-family residence, used as his or her primary home, to do "gas-fitting" without a license and without any permit or inspection (depending entirely upon whether your town enforces building code -- mine does not).

Of course, it is still required to conform to the "code" and the best way to ensure that is to hire someone who knows how, meaning THEY would need a license to install or modify "gas-fitting".

Do the people you see doing this spray the connection with soapy water and look for leaks?

you should do this even if you connect a bbq gas bottle.

Exactly what do you mean by 'hook up'?

If you are replacing a gas stove with a proper shut off valve and screw on connection with a new stove with a matching connection, it is perfectly safe, even if not technically legal.

The answer to your question isn’t as simple as your question. There are areas, as mentioned in other answers that require a professional to install an appliance. There’s places that allow the homeowner to install appliances. Ultimately, if you don’t feel that you can safely install and test an installation, you need to hire someone. If you have the ability to install and check for leaks, do it yourself, unless you live in an apartment, condo or some other shared dwelling where you could potentially run into problems. 

They are not suppose to do it themselves and gas, is not an area to screw around with. When I updated my kitchen a few years ago, I called a my plumber for the gas stove. Good thing too, becouse it had to be turned off at the meter, which then triggers an internal cut out that I had no idea existed. He did though. There are little things like that the ones in the trade know of. 

In my area you are required to have a gas dryer out in professionally. Many plumber are qualified to do this work.

with gas, even the smallest leak can cause a deadly explosion.  Thus, many rental places REQUIRE that you use a properly licensed plumber [not all plumbers are licensed for gas -- call the gas company to get their local list of properly licensed plumbers].  Of course, plenty of people evade the requirement in order to save the estimated $100 cost ... and if the place is a rental, they risk a massive lawsuit if they do it wrong and there is an explosion.  Yes, some knowledge and, preferably, experience is required in order to do the job right.  Plainly, you don't have that knowledge and experience -- yet. Suggest you pay and watch the work being done -- then you might have an idea of what to do the next time.  -- grampa