In gwTeX there is only one variable, namely PATH is set to /usr/texbin, and that is a symlink to the proper version of binaries (/usr/local/gwTeX/bin/i386-apple-darwin8.8.1 in my case). Nothing else is needed, even though Oliver might explain more details if needed.
If I'm not mistaken this not only applies to gwTeX but rather to all TeX distributions on the Mac once MacTeX is installed. Although MacTeX looks like merely a repackaged TeX Live 2007 at first glance, it does come with an additional management structure for multiple TeX distributions. This additional layer consists both of a GUI part and a command line tool and is by itself completely independent of TeX Live. Unfortunately it's not yet distributed as an individual package (although this might change in the future). Anyway, as soon as a TeX distribution for the Mac brings along suitable information about itself this management layer will be able to seamlessly switch from one distribution to the next by adjusting only the symbolic link Mojca mentioned. In case you're wondering, for most old distributions that shipped before MacTeX this information is provided by the switching layer. This mechanism is most probably the reason why TeX Live 2007 doesn't leave any trail of environment variables behind and also why this would be highly undesirable for any TeX distribution on the Mac. Oliver