Here's what it should look like:

Here's what it does look like:

This is Gill Sans Light.
This is Gill Sans.
This is Gill Sans Bold.

Results

Safari up to v62 displays Plain as Light.
Safari v64 displays Light as Plain.
OmniWeb 4.1 displays Plain as Light.
Chimera 2002-02-12 displays Light as Plain.
IE 5.1 displays Light as Plain.