What to do if i discovered out i had gingivitis yesterday my dentist said it was 5ml deep is this really bad?

Not good, not bad: A 5mm pocket needs attention... exactly what attention it needs is up to your dentist. Will s/he do a deep cleaning first, and then see if shrinks? Will surgery be indicated? Can't tell over the internet. Ask your treating dentist.

5mm not ml: 5mm is sort of the dividing line between just inflammation in the gums (gingivitis) and early resoptive changes in the bone (periodontitis) pushing the limit how effective your homecare will be. Brushing properly and flossing effectivley cannot extend beyond that 5mm depth in a pocket so that number indicates likely need for deep cleaning, re-evaluation in several weeks homecare modification.

See a Periodontist: I'm sorry you are going through this. The good news is if you get it treated properly and you keep good oral hygiene your may keep your teeth till the rest of your life. You need to make an appointment with a Periodontist, a dentist who specializes in gum disease, and start a treatment plan ASAP. It will improve your oral and general health.