Case 1. When this is your own code:
In general, you only need to put Copyright notice in the work (here source code). The license doesn't require a mention.
For example, for Apache license doesn't need any such reference.
However, GPL specifically wants that you reference
GPL License word inside each source code.
the process involves adding two elements to each source file of your
program: a copyright notice (such as “Copyright 1999 Terry Jones”),
and a statement of copying permission, saying that the program is
distributed under the terms of the GNU General Public License
In this case, to avoid the interpretation that the code is uncondtionally GPL'ed, it makes sense that you either specify additional clause like
"subject to terms and condition provided in LICENSE.txt"
Case 2: If you are modifying the other person code:
You cannot do -dual license, and also you cannot modify anything on the source that provides licensing information.
BTW: There is one simpler alternative that is obvious.
If you make identical copy of the code which has no mention of GPL at all and you use that copy for the Commercial or alternative license.
But this might just be too cumbersome.