Gastroscopy found 1cm sliding hiatis hernia. Is this serious?

Can be if you reflux. Hiatal hernias often cause GERD, also known as reflux. This feels like heartburn, and over time can severely damage your esophagus. Omeprazole is a good medicine to lessen the symptoms of GERD, but your best bet for the long term is surgery. Get it done while you're still on your parents' policy.