I'm not really sure how to do this in a point and click friendly way although
you could create a tar.gz file (probably a right click on the folder to
create in nautilus or whatever archive program you use) and then use GPG to
encrypt.
As far as whole disk encryption goes the consensus from free software privacy
projects like Tails seems to be to go with LUKS and use the Gnome Disk
Utility. Of course I'm not really sure how this works with Trisquel 5.5. I'm
imagining the Gnome Disk Utility is not installed in Trisquel 5.5 and I don't
know if you can get it separately in a sane way. I think this is also really
applicable to Gnome 2...
Take a look here for some directions that you shouldn't be able to follow due
to the changes in Trisquel's user interface although might still lead you in
the right direction on finding it in Trisquel 5.5:
https://tails.boum.org/doc/encryption_and_privacy/encrypted_volumes/index.en.html#index1h1
