Here's another idea. Take the post out and make a composite post by
splicing it to a length of treated wood post. To make the splice, cut
the bottom of the redwood post into an upside-down U shape, and cut the
treated post so it has a tongue that sticks up and fits into the U.
Assemble with glue and wood screws. I think if done well it could be
fairly inconspicuous. I would guess the splice need only be 12 or 18
inches long. More trouble than I would go to for a redwood post, but
you seem to be willing to put time into it. Whether this would stand
up to the stress of being a gate post might be a little dicey.

-- H