p 58 in the 1703 edition of Joseph Moxon's Mechanick Exercises states at the top of the page:" The Flemish-steel is made in Germany, in the Country of Stiermark and in the Land of Luyeck; From thence brought to Colen ( Koln ?) and is brought down the River Rhine to Dort, and other parts of Holland and Flanders, some in Bars and some in Gads, and is therefore called Gad-steel. It is a tough sort of Steel, and the only steel us'd for Watch-springs. It is also good for Punches; File-cutters also use it to make their Chissels of, with which they cut their files. It breaks with a fine Grain, works well at the Forge, and will take a welding heat."
This sounds like a blister steel with the carbon content about the same as a water hardening spring steel. It would make a tough blade and could be forged thin and whet thinner.