Tarnish / Oxidation on Tuner Gold Plated Faceplate

You probably can't. There are two reasons. First, the manufacturer will often have the faceplate goldplated over brass, or another metal which has been nickel or copper plated to be able to accept the gold plating. This gold plating is very very thin on a molecular level, and therefore cannot prevent air from oxidizing the metal beneath the plating. Knowing this, some manufacturers will put a clear protective coating over top of the gold plated surface. Unfortunately, if that coating is worn or damaged in any way, oxidation will occur. This will often look like brownish spots underneath the clear coating, radiating from a pit or scratch.
If it is important to you, there are plating companies that will strip and replate it. You just want to be sure that the quality of how it is done, and gentle use will allow it to remain looking good for a longer time.
It is surprising that you see this happen on some very costly gear, like the gold logo on some big MBL amps.

Thanks Roxy for the comprehensive explanation and information pertaining to tarnishes and oxidation on plated surfaces. With regards to stripping and re-plating the faceplate I think it is going to be a challenge as the existing black letterings and wordings (not sure if they are silkscreen letterings) on the faceplate will also be removed during the stripping process. Unless these letterings/wordings had been embossed on the faceplate, they will be gone forever after stripping and re-plating.
Well, I guess I will just have to leave it as it is and wipe it with a clean microfiber cloth occasionally. Thanks again.

Try Weiman glass cook top cleaner. I use it on my chrome amps to remove tarnish and oxidation and it does a great job! I think Walmart sells it.

Thanks for the information Yogiboy. Will try to look for it.