Then why not call it LOSS? The F has to mean something, and if it doesn't mean libre it either means gratis or it's redundant and confusing. GLOSS could be a different thing, to differentiate it from PLOSS (paid libre open source software).
It is in opposition to "restricted" software where you are restricted in all of those. (Copyleft software restrict others from adding more restrictions, which is defensible but some BSD proponents disagree)
Libre is, as many have said, just the Latin word for Free. You'd still have to explain what it means in this context.
"Open Source" has similar problems: many think that it's enough for source code to be available for inspection for software to be called "Open Source", but it is just a another attempt to market a philosophy under a particular name.
These constructs are created to have a special meaning to promote an ideology, and are thus usually capitalized in English. This is nothing uncommon, and attempting to use these—now accepted—phrases to mean something else is bound to cause confusion. So, just like you learn what a "programming language" or "HyperText Transport Protocol" is (yes, one transfers a lot more than just "HyperText" over it), if you are in IT, one should learn what "Free Software" and "Open Source" mean.
I do like the idea of distinction between GLOSS and PLOSS, but we've got enough acronym soup at this point that I'd say just stick with FLOSS.
Also, many people who only speak English don't know the word Libre.
Because then people would think they are losing something by using it? /s