Best way to vent gas from gas lines in preparation for opening gas line
Do not use pipe dope.  The secret to making gas tight fittings is to screw in each connection until only one or two threads are showing.  Yellow Teflon tape makes this easier, but it is not a requirement.  It can take up to 24 hours for the steel pipe to seal up, so do not expect it to immediately be gas tight.  A 16 inch (or longer) pipe wrench also makes this easy, but it is still quite a workout.
Yes, there will be natural gas in the line.  It should be at a pressure of about seven inches of water column (equivalent to 1/50 atmosphere, 0.02 bar, and 0.25 psi).  If there is a really long and fat pressurized pipe (50+ feet of 3/4 inch) in the system, opening could release a few cubic inches of gas.  But it is going to rapidly dissipate and float away.  Natural gas is buoyant; propane sinks and accumulates in low places.   Personally, I wouldn't worry about gas in the line.  Just opening a nearby window is more than enough.  (Of course, I don't smoke either.)


There is no reason for a plumber to lay pipe.  This is something anyone can do.  Homeowners are, in my experience, more likely to do a better job because the professionals don't have to live with the installation.

As long as you do a good pressure test, you can assure yourself and your loved ones that it was done well.