Note that this isn't an issue with just default Windows fonts installed, as they don't include Gill Sans. But it's provided by MS Office products, so it's likely that a lot of users do have it.
So the background to this can be found in bug 551313. The Gill Sans fonts provided by Office on Windows are organized weirdly, such that the "usual" faces (regular/bold, italic and non-italic) have the family name "Gill Sans MT", but then there are also UltraBold faces with the family name "Gill Sans". This causes a problem when a site (like perfherder) says
font-family: Gill Sans, Gill Sans MT, ..., aiming to support both macOS (where the family name is just "Gill Sans") and Windows, but ends up getting an UltraBold-only family on Windows unless we hack around it.
The above patch is only a partial fix, hiding the UltraBold-only family, sufficient to avoid the bad rendering reported here. For a complete "fix", we should really merge the UltraBold faces into the Gill Sans MT family, but that's going to be more complex and may not really be worth the hack.