I live in a qth which is very prone to thunderstorms in winter.
Which are the best arrestors, spark gap or gas filled ?
With either one rated for higher transmitter power on the antenna side of a tuner , it is very unlikely you will protect very well. The gas type is better and more reliable, but none are very effective if they pass transmitter power. This is especially true on the antenna side of an antenna tuner.
Your best protection is a disconnect system of some type, along with bonding of the RF cable entrance to the power mains entrance grounds.
If a gas filled flashes, does it need to be replaced ?
After a while with many medium hits, or with one strong hit, they can go bad.
Which type is ideal for a remote atu ?
None. It isn't that simple at all. It is fairly difficult to protect a remote tuner, other than a full disconnect..