Do you have a parts book? If not, look at the diagram on this page:
https://www.circlegtractorparts.com/hydraulic-steering-and-pipe-4wd-18-28hp-tractors
The input shaft seal in part #17 is leaking and forcing oil up the outer tube part #6. The leak is actually at the bottom of the column. There is no parts breakdown for the steering gear box. To fix it your only option is to take it apart and take to seal to a bearing supply place to see if they can measure it for fitment.
Personally, I would buy an entirely new gearbox as there may be other parts worn. Then once the new box is on and working, I would take the old one apart to see if it is worth fixing for a spare to have on hand.