You've mentioned GoJump/Gransee. It is a great DZ near Berlin and they operate 5 Days the week (Tuesday and Wednesday off). The Dropzone is really busy with lots of tandems and they operate at least one PAC750, sometimes two.
If you stay near Berlin, you should also give Funjump/Fehrbellin a try (Mondays off). Not as many tandems than Gojump, but really nice people floating around even during workdays. Funjump operates a C208B Supervan. And if you're in freeflying, you have no other choice than visiting Fehrbellin during the famous Skyparty in mid august :)