Developers like Edwal Super 20 fell out of favor with the advent of newer finer grained emulsions. Paraphenylenediamine is a slow acting developer and a silver halide solvent. Its action in Edwal Super 20 is primarily that of a halide solvent. When used alone this developing agent results in a severe loss of film speed which explains the presence of Gradol to partially counteract this loss.
There are two downsides to the use of phenylenediamine. First, modern emulsions may not react favorably toward it. Second, it can cause severe allergic dermatitis and is a known cross sensitizer. Which means that you may also develop a sensitivity to Metol and the color developing agents. If you insist on using it wear a dust mask when handling the solid and protective clothing and nitrile gloves when handling it or its solutions. I can attest to the fact that it is nasty stuff.