I find a wire brush is usually enough to clean threads.
Pipe dope and/or teflon tape
Yes, first the Galvanized pipe has to be threaded with a standard thread for pipes, then you buy the correct female socket threaded plastic fitting, screw it onto the pipe using Thread pipe sealing tape, then glue your piping into the socket part of the plastic fitting, simple. Cheers.
3 to 5 turns by hand then 2 by wrench
GI Pipe is "Galvanized Iron" Pipe. These days the pipe will likely be made from steel rather than iron.
You can use pipe thread sealant aka "Pipe Dope" you can get this at most hardware stores.
Yes.
ya the galvanized pipecan be use in undergrounds,,, because it is an iron pipe
Yes
Galvanized Iron pipe
galvanized iron pipes commonly used on water pipe lines, it is dipped into zinc for corrosion protection
Anytime you disturb galvanized pipe you usually have some rust or buildup come loose. This will either stick in the valve or clog the shower head. Depending on how old the galvanized pipe is and how hard your water is you may have to clean the control valve several times.
No, do not use glue. Use teflon tape or pipe dope on the threads. You should also make sure you always put the male PVC thread into the female metal thread. If you do it the other way you will tend to split the PVC fitting when you tighten the threads.