Gel strength, also known as 'bloom' value, is a measure of the strength and stiffness of the gelatin, reflecting the average molecular weight of its constituents, and is usually between 30 and 300 bloom.
Powdered and leaf gelatin are usually interchangeable, but for best results try to use the type specified in the recipe.
As a general rule, two gelatin leaves is equivalent to one teaspoon of powdered gelatin.