This is a question I hear often when our clients are doing some spring painting and decorating.
If the existing wood has clear, flaky pieces coming off it, then it has been varnished in the past, most lightly polyurethane varnish, which works well on wood especially boats if one maintains the wood regularly, by gently rubbing down the surface without breaking existing varnish, and just adding another coat.
This way, layers upon layers are building up to protect your wooden gates or windows. But the most important thing with this method is doing it regularly (once a year), for once the seal is broken and the air and moisture goes into the wood, the grain will turn grey over time and the varnish will go flaky, then you have to adopt a new approach. The photo here is a gate which was well overdue some attention. We stripped and sanded it back as much as possible and applied an oil.
Varnish in the sun; if you have a south-facing gate or windows then the sun will undermine your windows over time. I find that a good furniture oil (see products we recommend ) will do a great job, one can also strip off old varnish and apply oil, but if the wood has gone grey in the grain, then it may be there to stay depending on exposure, (see photo still a good finish though). But the oil will still bring up the wood very well. The one thing about oiling wood is when you oil the wood, there is nothing there really except oiled wood, so after time one just cleans whatever surface one wants to re-oil and apply the oil, to bring wood back to former glory. Again, for best results it is recommended to do this at least once a year for maximum protection. You can get UV protection in the oil to prevent your wood from fading. I must admit, I do prefer oil over varnish or stain varnish, as if you forget or decide to skip a year then the preparation is easier, as one does not have to strip flaky varnish. You just sand the wood, wipe with white spirits and re-oil. For best results follow the instructions on the tin.