Bb13b9 can only be one thing, but that can be made more clear by using parentheses: Bb13(b9).
It is common to put altered extensions and added notes in parentheses at the end of a chord name. Also note that altered extensions are typically written in ascending order, so an altered B7 might look like B7(b9b13), or even B7b9b13, but rarely (if ever) B7b13b9, or even B7(b13b9).
In the absence of a 7th, with unaltered extensions, you might have Bb6/9, but a triad with added altered extensions would be pretty rare.
Similarly, Gb13 can only be one thing. A G chord with an added b13 would be notated explicitly as an added tone chord: G(addb13) or G(addEb). But G(addEb) would be an uncommon chord since altered extensions are usually added to seventh chords, but not to triads. So a Gb13 is just a G13 transposed down a half-step.
But, note that Gb13 is neither of the options suggested in the OP: "is it a Gb with a 13 or a G with a b13?" A G13 is a G7 with added 9th, 11th, and 13th extensions. It is very common to omit the 9th and 11th (and possibly the 5th), but the 7th and 13th must stay. So a G13 is at least G7 with an added 13 (E), and a Gb13 is at least Gb7 with and added 13 (Eb).