If there's a diagonal you should be fine. It'll all expand and contract as one. Larch is fairly stable, too (after it's dried, some boards get twisty in the drying process).

The thing is the gate will be hung on 2 brick pjllars so any shrinkage or expansion of the gate may be problematic. I already hung larch window shutters, and they tended to shrink a bit - but in this case the vertical boards were.one next to the other.

Wood expansion/contraction occurs predominantly across the grain, not in the lengthwise direction, so your gate should be fine over the long term. To control sagging, though, make sure you have a diagonal brace, and I would use a dab of a good polyurethane construction adhesive at each overlapping joint.

Actually, I was considering going ahead with the all larch gate, and if it sagged, twisted or expanded / shrank too much, I could always reuse the vertical boards, and attach them to a metal frame?

I usually just use 2 screws for overlapping boards, stainless screws in this case and probably bury them slightly, using some homemade larch sawdust plus wood glue to cover them, or would you say that gluing (with wood glue?) overlapping boards (using one screw) would be better? Thanks