Would he have done it?

[Aiwendil]

Fordim wrote:

Eru doesn't really appear in LotR -- he was a creation of the "later" Tolkien

This simply isn't true. Eru/Iluvatar existed as early as about 1920, when Tolkien wrote the first version of "The Music of the Ainur". It's true that the final version of the Ainulindale was not written until the 1950s, but the story was not much changed in its essentials (certainly far less than many other parts of the Legendarium).

"Or are you saying in the context of LotR alone?" -- what other context is relevant here? The question as posed is whether or not Gandalf would have taken the Ring; such a situation exists only in the context of LotR.

The other context that may be relevant is that of LotR + the rest of the Legendarium.