Given the purpose and original meaning of “gargoyle”: The term originates from the French gargouille, which in English is likely to mean “throat” or is otherwise known as the “gullet”;[2] cf. Latin gurgulio, gula, gargula (”gullet” or “throat”) and similar words derived from the root gar, “to swallow”, which represented the gurgling sound of water (e.g., Portuguese garganta, “throat”; gárgola, “gargoyle”).
I’d say that is perfect. Giger’s creation is most famous for its relationship to throats (the “hand” stage attacks through the throat, and the “adult” stage has that terrifying projectile throat). Not to mention the creature is certainly frightening enough to keep any evil spirits at bay. Who cares if it’s not “period”; it fits the spirit of the gargoyle perfectly. As another noted above, I’d pay good money for one, were I in the market for a gargoyle, and further, I’d support any effort to “officially” elevate the xenomorph to legitimate status as a modern addition to the catalog of gargoyle types.
Should add too that gargoyles are supposed to be elongated, in order to “throw” the water farther from the building, and the xenomorph’s head is uniquely suited to that purpose by dint of its shape.