The Germanic tribes shared much with their Celtic counterparts, including diet. Much of what they ate was farmed, including grains like barley and rye for bread and beer, along with various vegetables. Livestock included cattle and sheep, so you would expect dairy products, naturally. Your average meal would consist of perhaps a gruel like the Roman puls or a stew. I have an AJA journal article in the hell-pit (spare room) somewhere that I believe has more on the topic, and if I can find it for you I will post it.

Sorry, why did OP ask about 77 BC? Is there particular significance to that time?

Julius Caesar gives a brief description in his Gallic Wars and accounts for the eating practices of a particularly militant group (the Suevi). In Book 4, Chapter 1 he states that "They do not live much on corn, but subsist for the most part on milk and flesh, and are much [engaged] in hunting..." I am not certain if this accounts for the varying seasons and whether or not this diet was more relevant during wartime. The introduction to the chapter leads with a wintertime setting.

Despite this, earlier in his writings he mentions the Helvetti preparing loads of "corn" in many wagons. Perhaps some groups would have had more (peace) time to farm largely determining what you ate.

edited: quotations on corn, unsure of why the translation spells it this way if referring to the German "Das Korn" meaning grain.

C. Julius Caesar, Gallic War, Book 4, Chapter 1 http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0001:book=4:chapter=1&highlight=milk