Google Books has been scanning books that are older and older. Now you can read full scans of 17th century books, such as:

D. Joannis Gerhardi homiliae XXXVI seu meditationes breves diebus dominicis atque festis accomodatis. These are brief meditations on the Gospels for most of the Sundays of the historic church year.

Sacrarum homiliarum in pericopas evangeliorum dominicalium et praecipuorum totius anni Festorum, Volume 1. These are full length sermons. I have not yet compared them with the Postills published by Repristination, which were translated from the 19th century German editions. These homilies can serve as a great entry into Gerhard’s Latin. The style is straightforward and lacks philosophical jargon and references to outside literature.

Sacrarum homiliarum in pericopas evangeliorum dominicalium et praecipuorum totius anni festorum, Volume 2. This is volume 2 of the above.

Happy reading!