Are the new replacement Gretsch bar bridges suitable for a mid 60s Gent ? (with a little relicing to match the other parts)

I don't think the spacing will match and I think they're built differently. Yours will have rocking posts with the bar lying flat on the thumb screws and on the new ones the bar itself is rounded at the bottom. So it rolls over the thumbscrews.
But of course if you get both the bar and the base you should be fine. If they're the same height, which I don't know.
I know that Proteus makes a Tru-Arc low rider that might be the right height. But if you send him measurements he'll tell you more.

I tried a modern Gretsch bar bridge on my 64 Gent and it was too tall. I bought a low-rider Tru Arc SS from Proteus and it worked out great. However, I sent him my original bridge and he made my Tru- Arc to match the string spacing.

Gotta give the nod to Proteus and Tru-Arc. Here's evidence that he can make whatever you want: