You can look it up in Ilford's data sheets, which include an ISO contrast range specification for each contrast grade. For example, for Galerie FB, grade 2 is specified as 110, and grade 3 as 90. For comparison, for the new MG FB Classic, printing with Multigrade filter 2 is specified as delivering an ISO range of 95, and filter 3 as 80.
There are many caveats to this. Contrast range is affected by choice of developer. Different brand or vintage filter sets or dichroic heads may behave differently. Most importantly, the shape of the paper's characteristic curve is as important as the ISO range in determining the tonal scale of a print - two papers could have ISO range specifications that match for all grades, but the negative might work better esthetically at grade 2 on one of the papers and grade 3 on the other, just because the two papers twist the tonal scale in different ways.