The brothers Richard and Hugo Gruhn each had their own motorcycle firm, both based in Berlin.
1. Richard Gruhn founded his company 1906 and built motorcycles and engines until the early to mid-1930s. Gruhn engines were fitted to ABC GmbH, AMBAG and other motorcycles.
2. Hugo Gruhn operated from 1920 to around 1927 producing frames and chassis components for motorcycle manufacturers. He also marketed a lightweight motorcycle kit which included engines from Cambra, Hanfland and Diag. DKF is listed as using engines and chassis from Gruhn, so it is quite possible that both brothers supplied components for that firm.
N.B. Tragatsch says Richard's machines had "no sporting image". GTU says completely the opposite.
Sources: GTU Oldtimerservice, Tragatsch