A gentleman's agreement or a gentlemen's agreement is an informal agreement in which people trust one another to do what they have promised. The agreement is not written down and does not have any legal force.
What does this mean for your business? It means a complete fatality and downfall of everything that you have worked hard for. You have no legal protections. Your business partner(s) can and will rob you with no legal recourse. If you are sued by a customer, all of your personal assets (bank accounts, home, car, etc) are on the HOOK!
Please think long and hard before entering a gentleman's agreement when starting your business.