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What province are you in please?By gentleman's agreement do you mean this was a verbal agreement and there was nothing in writing?
I am saddened to hear of this story and your difficulties.
I suggest you send him a letter by Registered Mail and in the letter give him a deadline for reimbursing you in full.
Tell him that if he fails to comply you will commence legal proceedings and then will be seeking interest, damages and your costs.
If he doesn't pay you, you can sue him in Small Claims Court and can self-represent and if it gets that far and you find you need some help as you go through the process you can always post on this site again and I can assist you further.