ArticlesPlus, on the Start Here page of this Guide, is an excellent place to look for critical studies on German language or literature. Among other sources it includes the contents of the MLA International Bibliography, which is the major database for research in Modern Languages. If you wish, here is a link to access the MLA International Bibliography only.
Listed below are three different, though similarly-named, sources for literary criticism. There is a lot of duplication of contents between them, but each one does have sources that the other two do not, so they're all worth looking at.
Gale Literary Index allows you to locate information on authors from all times and places that are included in more than 100 publications, both print and online, from Gale and other publishers. Search results include the contents of Dictionary of Literary Biography.
It does not provide the full text directly, only pointers to where the full text is located. You will need to use the ALICE Catalog to look for the specific titles that you find listed in the Gale Literary Index.