I'll start by stating Debian is not on the FSF-certified list of free
distributions. It gets very close and it t might otherwise be today if not
for a few issues related documentation and a non-free repository existing
within the project. I mainly say this to make sure your aware of these issues
before proceeding. The issue you have is probably not specific to Trisquel or
Debian.
My gut feeling is your installing two different distributions that are
dependent on different versions of the bootloader (be it the same bootloader
or a different one). If the distributions are installed in the right order
there probably isn't a problem. But because you've installed it in the wrong
order the older bootloader is overwriting the newer one and the configuration
files for it aren't properly recognized by the older version.
There are different ways to solve this problem. During installation of Debian
you are probably being asked where to install a bootloader (GRUB probably)
and the default is to install it to the internal hard drive. Instead of doing
that install it to the external drive where Debian is being installed.
Then when your system starts up press the F12 key (this is for your Gentoo
laptop only; other systems may have other keys to bring up the boot menu) and
select the external hard drive.
You should now be able to boot both Trisquel and Debian/or whatever other
distribution you want.
