Under normal circumstances 18 minutes are enough in Berlin to change trains but it does not give you any buffer in case of a delayed train to Berlin. I guess there are only a view trains going from Berlin to Gdansk per day and missing it would cause you some long delay. If possible you should try to take a train earlier to Berlin to have between 30 and 60 Minutes. About the validity of your ticket. If the whole journey from Freiburg to Gdanks is on one ticket than in case of a delayed train you can take the next train to Gdansk. You just go to the Bahn Infopoint and get a note about the delay that caused you to miss the connection (this is the official way, many people just skip this and just take the next train which can theoretically cause some debate with the train staff). If you have separated tickets the same rules as in air travel apply meaning if you miss you connection you out of luck.