Turkish Airlines Flight Boarding Denied 30 Minutes Before Departure

I purchased a ticket from Turkish Airlines. We arrived an hour early for the morning flight on June 13. They informed us during the check-in process that the gate had closed 30 minutes before departure and denied us boarding. We lost both our hotel and flight payments. They directed us to a place where we received a very small amount of compensation. This small amount wouldn't even cover a ticket for one person, let alone six. Additionally, the seats we had paid extra for were changed; we saw this in the system before the flight. I want proper compensation. I do not want to be called; we already heard the same things in the morning. I wonder if our tickets were sold to others. If you respond, please do so via SMS or email. I want either a voucher or a discount. If not, I want a full refund. It's not a small amount.