Teak cleaners tend to be acid-based. While it’s safe for gelcoat
, I personally wouldn’t apply it to the entire deck. Aggressive cleaners can increase the porosity of the gelcoat which traps more dirt over time. So while it will look great initially, it actually accelerates degradation.
Starbrite does a deck cleaner with PTFE that’s quite good and it fills in the pores so it slows the eventual degradation. If you really want to level-set the tone and get it back to the same bright white as the stain, try applying Fiberglass Stain Remover (FSR), but be careful with that as well, it’s pretty potent stuff. Once you’ve done that, I’d recommend regular application of the Starbrite deck cleaner.