Helen
I don't think Gilson published anything - my main source of info is the T&TGW volume which I don't have here. He did write somethings though. I'll post what I find later.
As to 'Truth', I suppose whatever it is, its on the other side of those 'Mountains' that Niggle set off towards, so we on this side can't really know what it is - yet. But unless we believe that, whatever it is, its really there, we won't even set off to find it. I suppose someone could come from over the mountains to tell us about it, & show us the way to get there (or perhaps
someone already has
).
Perhaps 'enchantment' is the first step towards this Truth, the thing that lures us in - like Niggle's vision of the Tree which he attempts to set down on canvas, & then his finding of the real Tree is the eucatastrophe of his story, & the eucatastrophic experience, the flash of 'truth' which it reveals, inspires him to go on over the mountains to find the truth itself. So the sequence would be enchantment - eucatastrophe - 'Truth'. Idea just occurs - could it be a kind of 'harmonic' sequence - so 'Truth' reflects 'downwards' into eucatastrophe, which in turn reflects downwards again as enchantment, like 'pure' light shining through lenses (or 'planes'). Enchantment makes opens us up to the possibility of Eucatastrophe, & Eucatastrophe is a sudden flash of 'Truth', of 'joy beyond the walls of the world'.
Which would mean that Tolkien's intention with his Legendarium was to enchant us, & make us at least susceptible to the Eucatastrophic experience, & so inspire us to seek the Truth.
(Sorry, that's all just popped into my head & I don't know if it works - I'm sure someone will let me know if it doesn't
)