Melatonin need increases as we age. I have read that people should start taking 3mg melatonin at age 40. At age 60 that need increases to about 6mg. I’m 69 and taking 6mg is adequate for me. You can take more. Some people find that strong melatonin leaves them with a sleepy ‘hangover’ in the morning, But melatonin is cleared from the system by exposure to blue-containing daylight in the morning. So if sleepy… open your eyes and get some daylight. (That’s why blue ‘night lights’ are terrible for sleep and even painful on the eyes at night)
For GABA, I’m taking a single 500mg pill nightly. I take both melatonin and GABA shortly before going to bed every night.