He’s the only one that can be considered an authority on how the word is pronounced LMAO.
He’s just the guy who invented the software and coined a name for it, he has no authority over how that should be pronounced. If he came up with a ridiculous pronunciation (as he did) he should be laughed at and people should use a sensible pronunciation.
Pronunciation isn’t based on spelling
Of course it is. That’s how spelling works. In English it isn’t nearly 1:1 like other languages, but spelling is very strongly tied to how a word is pronounced.
the English writing system isn’t consistent enough to make estimations for a pronunciation like that
Yes, it is. That’s why people pronounce it with a hard “g”, because they’ve internalized the rules for spelling vs. pronunciation in English and know that those 3 letters in that order has a hard g.
are pronounced wildly phonemically differently
There are slight differences in pronunciation, not wild differences. The differences are so slight that normally you can understand the word someone is using in another dialect without difficulty. And, in every English dialect “gift” has a hard g, as does “gif”.