For polyester resin and gelcoat…
…you absolutely should wear an organic vapor respirator. The fumes from these materials are strong and toxic. There are several good brands, but the most commonly available ones seem to be AO Safety and 3M.
Once the products cure, you can switch to a high quality dust mask for sanding. I prefer cartridge style masks and you can just swap dust cartridges for the organic vapor cartridges on your respirator.
I don’t use an organic vapor respirator when working with epoxy, but some people are sensitive to it and need the extra protection. It certainly doesn’t hurt.