The best place to obtain Audacity is from the Audacity web site as this always provides the current version http://audacityteam.org/download/
The Nyquist Noise Gate effect is available here: http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Noise_Gate
Installation instructions: http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#Installing_Plug-ins
Note that a noise gate does not reduce noise while sounds are playing, it simply reduces the noise level between sounds.
See the help file in the “download” ZIP package for details about how to use the Noise Gate.
For details about using the Noise Removal effect, see here: http://manual.audacityteam.org/o/man/noise_removal.html
For additional help with reducing noise, see this article on the wiki: http://wiki.audacityteam.org/index.php?title=Noise_Removal