If I create from scratch a software tool, but I want to use GMAKE (vs FreeBSD "make") to build my tool from source, does using GMAKE prevent me from license my tool under a BSD license since my tool is built using must use GMAKE vs FreeBSD make?
IANAL (and the GPL has no clear legal precedents on what constitutes a derived product), but you should be able to license your product however you like (at least wrt gmake), provided it's not a derived product of gmake. Unless you, for some reason, link to the gmake source code or something similar, you should be fine, and could ship both source and compiled code.
On the other hand, if you want to ship gmake bundled with your software, you must follow the distribution parts of the GPL. Afaik this includes some notes in your documentation and supplying a copy of the GPL. Any modufications you were to make to gmake would also have to be released under (I believe) the GPL.
Unless you are actually using gmake as a part of the final project (as in, its source code, its documentation, or its letterhead), then it is not a derived work, and so you can license it however you want.
Think of it this way, a derived product is a product which takes a substantial portion of its parentage from the product it is derived from. Ice Weasel is derived from Firefox (clearly, as Ice Weasel is little more than a skinning). On the other hand, if a tool is used for a project, even if the very existence of the product is rooted in that tool being able to work, it is not necessarily derived — most of my work (these days) is done on a Mac, that does not mean that Apple owns my software.
A compiler is a tool, not a parent. While you may build something which depends on gmake exclusively, it would still not be derived as someone else could then come along and write gekma (an anagram for gmake) which behaves exactly the same but shares none of the code base (Linux/Unix much?), and your software would work with that too.
Now, if you were to create
gmake2 which took gmake and added the ability to, say, automatically turn on the coffee machine while compiling (after all, compiling is work! You might get thirsty!), then that is derived as you are still using gmake as the basis of the project. No matter how many gekma's and kemag's are created, your source code will still include the gmake source (unless you remove it).
For more information, you can also look at derivative work in US law.