[Make sure the gun has no cylinder pressure before doing any maintenance- Shoot it until you are sure it is empty. Then check the barrel is clear and it is uncocked].
Old type fill valve assembly (photo shows cylinder end plug with barrel band removed).
1. If the leak is between the quickfill nipple and end plug, it may be as simple as very slightly tightening it to compress the bonder washer a little more. If that works it's still worth replacing the washer which may have compressed or deteriorated. (Remember to depressurise the cylinder again!)
Do not overtighten. If that fails...
2. Using the correct size spanner on the fill connector should allow you to unscrew the cylinder end plug, exposing the two sealing o-rings. If the fill connector unscrews from the end plug you may have to use a padded vice to lightly grip the plug and turn the cylinder by hand when separating the two.
[Make a note of the force required to undo it, use this as a guide on reassembly].
Take the old items (bonded washer and o-rings) to a hydraulic fittings company, if you aren't sure of the size.
Replace and reassemble, do not apply any lubricants or grease to the threads.
[If the cylinder will be "left open" for any time, cover the opening with tape or clingfilm to prevent debris/contamination].
Hope that makes sense and is of help. ATB