Are pentesters required to obtain licenses for open source software they use?

For example, if you were doing a security analysis for Company A, and part of your analysis involves using Nmap or Wireshark, etc., are you required to obtain a license from the vendor, or do you use the software under the GPL?

I cannot find anything saying wether or not software licensed under the GPL can be used for business.

That is actually a rather good question.  I think I'd go mainly with what Shakuni said, and check the specific license info on the product install.  That said, most GPL programs like that... I'm fairly possitive are kosher to be used, and ment to be used, in whatever fashion you have.  As long as you're not profiting off selling their software, I don't think you're busting any rules or such.  I would double check on that of course.  But that's a bit of the difference between GPL open source and say... free-ware or shareware.   As long as you're not trying to break the GPL cycle, and sell the software you are good.  It's not like AVG where it's free for home, but you'd best buy for work.  If you're using the program as part of your professional career I am 99% sure it is fine and Dandy.