They say the money was a gift. How do I prove it was a loan?

Nothing decides more claims between people who know each other, and it is winnable in one place only: the moment the money changed hands. Everything created then is evidence. Everything created afterwards is characterisation, and a judge discounts it from both sides equally, because by then each of you has a reason to remember it a particular way. So the question is never what you both say now. It is what exists, dated, from before there was a dispute. A payment note reading "loan" is worth more than an hour of testimony. A reply saying they will get it back to you next month effectively ends the enquiry.

Where the record is thin, two other things carry real weight. The first is repayment behaviour. Any amount returned, at any point, is conduct that contradicts having received a present, and it is the hardest fact for the other side to reframe. The second is proportion. Judges apply ordinary sense to whether a gift of that size is credible between two particular people, and a large round sum between casual acquaintances rarely reads as generosity. Money between close relatives runs the other way, since gifts inside families are genuinely common, so expect to need something more than your own account of the conversation.