It can be difficult to predict the weather in Germany. It largely depends on when you're going and where you're likely to be. Here's a loose guide that should give you an idea of what to pack, whenever your gap year takes you to Germany.
The climate in the north and northwest of Germany is Oceanic, which basically means that it rains all year round. You should definitely pack a jacket if you're visiting this area. Summers are relatively cool, but can still sometimes reach 30C. Winter is also relatively mild, but it still gets pretty cold, so be prepared.
A simple rule of thumb is that the weather gets more extreme the further east you travel. This means that during the summer it can be very hot for long periods, while in the winter temperatures drop low and stay there. That said, these parts of the country are well-prepared for such extremes (and you wouldn't want to miss out on Christmas markets).
In these regions it can be a little more difficult to predict the weather from day to day. The south is commonly characterised by beautiful, sunny days that promptly erupt in torrential downpours. Generally it's hot in summer and cold in winter, so pack accordingly, and always be armed with an umbrella.
The weather here is largely what you'd expect from this area of Europe: unpredictable. Noticing a pattern here? Again, if you're travelling in summer expect it to be exceptionally warm but with the chance of showers, and dress warmly if your gap year in Germany coincides with winter.