Try the following link for information on hot dip galvanising:-

www.hdg.org.uk/specifying/standards.htm

US Steel has a good web page for galvanizing grade reference:

http://www.usx.com/corp/sheet/coated/hotapp.htm

A60 and A40 are ratings for galvannealed steel.  This is a hot-dipped process where the steel is heated after coating with zinc, converting the zinc coating into a harder zinc-iron alloy coating, that is also smoother, more scratch-resistant, and more paintable than a galvanized finish.  

Ordinary galvanized steel has only a thin layer of the zinc-iron alloy, at the interface of the steel and the zinc.  The outer coating remains zinc.

I believe that the A60/A40 coating thicknesses are equivalent to the G60/G40 galvanized coating thicknesses.

Excellent site, Mattman, thanks.

pylko...the designations "G60" and "G40" refer to hot dip galvanizing average weights of 0.60 ounces per square foot (both sides) and 0.40 ounces per square foot respectively.  The metric equivalents are 60 g/sq.m and 40 g/sq. m respectively.

ASTM Standards A525 are the specifications for hot-dipped gavanization.  My version is for 1987.

Also refer to ASTM A446.

Distinction of G designation versus A designation is in the chemical.  G designation is a zinc alloy and A designation is for Zinc-iron alloy.  I am not an expert on galvanization so I hope you can find what you are looking for in the ASTM Specs.