Would recommend you get started now, especially for GC. I had a teacher for the first half the year for 3/4 who didn't speak a word about GC and DS, then we had a second teacher take over around August who panicked when he realised how behind we were for oral preparation. GC is mostly straightforward, just make sure to prepare responses that are more than one sentence, and get someone to test you with questions that are out of the scope of your typical questions like 'What are your hobbies?' etc, so you can prepare yourself for more unexpected questions.
As for DS, my teacher for the second half of the year gave us a lot of material to prepare, including for the 1-min intro and almost all the questions you could possibly get. My advice would be to ask or gather material from your teachers and have an in-depth knowledge of your topic. A day before the oral, my teacher called me and asked me the most random questions that wasn't included in the material he gave, and while some of it wasn't relevant, it really helped me to prepare myself for challenging questions the examiners might ask. However, I mostly prepared for this around August/September on a daily basis so don't be too worried about preparing for DS right now.