"NoGS," for "No GreenScreen," is a common notation allowed on HPDs. Unfortunately, it doesn't tell us very much.

It means that the substance is not present on any GreenScreen Specified Lists, i.e., has received a GreenScreen List Translator score of NoGSLT, and does not have a freely and publicly available GreenScreen assessment and resulting Benchmark score. 

There is little you can surmise from this. It is possible that the substance is highly hazardous for one or more health endpoints but has not yet hit the radar for the authoritative bodies. It may be that it has been well studied and is fairly safe. Many authoritative bodies only study substances for which they have an indication of potential harm. There are few attempts to affirmatively identify low-hazard substances.

To get more information on assessing chemical hazards for substances that have not been assessed, see the GreenScreen website.