The glass used is generally standard float glass for most window and mirrors. It has a slight green cast but find this isn't a problem as one doesn't notice this viewed from the front. There is a water clear glass called "starfire". I prefer fairly thick glass; usually 6mm as it feels less fragile. Sometimes, for artworks, I will get the edges bevelled. Use toughened glass where appropriate for safety reasons.
This has to be done thoroughly for trouble free gilding.
Glass can vary a lot in its characteristics. Old glass tends to wet better and is easier to work with than new glass. It has been suggested that sodium and calcium ions migrate to the surface when it is young. This process slows down with time and adhesion is possibly better on old glass.
Glass is cast onto a bed of tin, hence the name "float glass". This affects it so that one side is different to the other, even though you can't see it. Stained glass painters notice it takes silver nitrite stain differently. Silvering for mirrors is done on the non-tin side.
I have seen on some old glass a fine milky mottle. This mottling is integral with the glass and can't be removed. Tinted glass also makes life difficult for window signwriters. Try a test piece to see if there is sufficient visibility.
Clear acrylic sheet (Perspex) can be gilded onto instead of glass. I try to avoid using it though as the adhesion is not as good, especially with the heavier types of metal leaf. Another problem with plastic is you can't use whiting to clean it as this would scratch the surface.
One area I have looked into and am keen to develop is using kiln-formed glass. This method of shaping glass is becoming more popular and there are many possibilities open in decorating it with glass gilding. As well as decorating slumped glass is the possibility of fusing leaf with glass.
Glass can also be sandblasted or glue chipped before gilding