How to fix loose post
"How can I make it solid?"

Something isn't right in your post. An 80lb sack mixes up 2/3 cubic foot. 

Spec sheet: https://www.quikrete.com/PDFs/DATA_SHEET-Fast%20Setting%20Concrete%20Mix%201004-50.pdf

The volume of your 2' deep, 30" dia hole, minus the 9" post volume, is 9 cubic feet.

Here is the volume calculation

Short answer is at 2' deep it'll never be a solid 6' tall gate post on it's own. Sp personally, I'd pull it all out, dig the post hole deeper and pack it with 1/4-minus.

Dig four 2' long trenches 15" deep, extending from the 9" post at right angles to each other.  One pair parallel to your gate, the other pair perpendicular.  Sandwich the 9" post with a pair of 5' long PT 2x6's laid in the trenches that are parallel to the gate. Bolt together. Do the same with another pair of 5' 2x6's in the perpendicular trench. Fill with concrete. Per Isherwood, add some rebar.