Bah.... What they haven't figured is that there is a small group of Gaelic words which have what I call variable vowel length.
It means that depending on the context, these words can have:
2) long vowel
3) short vowel
4) no vowel
In a stressed context, you get the long (as a rule of thumb) and in unstressed environments you get the short ones.
The most obvious group to do this are those irregular verbs which write hiatus in stressed forms:
bhitheadh /vi.əɣ/ (Grade 1) - bhiodh /viəɣ/ (Grade 2) - bhiodh /vjəɣ/ (Grade 3)
bhithinn /vi.ɪNʲ/ (G1) - bhinn /vi:Nʲ/ (G2) - bhinn /vi:Nʲ/ (G3)
There are more however where this isn't written: mi, thu, e, i, sinn, sibh, iad, tha, bha... Not sure why but probably because its context dependent.
So, for example with tha you get:
tha /ha:/ G2 (I think I've heard this with hiatus even but not sure)
tha /ha/ G3
th' /h/ G4
G2 you'd get in an emphatic answer for example, G3 in a non-emphatic statement or answer and G4 when it's unstressed, for example when linking sentences. Not writing this follows the general pattern in the old spelling which hates redundant information cause context makes it clear. So there's no need to introduce thà vs tha really.
Same applies to bi in the future which has the first 3 grades:
bithidh /bi.ɪ/ G1
bidh /bi:/ G2
bidh/bi /bi/ & /bɪ/
As before, G1 in a stressed statement or answer etc.
So, gum bi as /gəm bi:/ does occur but only if it's intended to be emphatic. For example, if the intention is to stress that in spite of ooposition, it WILL be the case that...
Failing that, the spelling gum bì is both unorthodox and wrong in this position.