John Gust push fittings to make serving from Cornelius Keg set ups easier. When unscrewing the original barb assembly from the gas regulator ready to fit the John Guest fitting, there is a fair chance the fitting will have been sealed with loctite. This should unscrew with a bit of elbow grease. The use of plumbers ptfe tape is advised to help seal the new fitting.
3/8 x 1/4 BSPT simply screws into a standard gas regulator to accept a 3/8" push fit gas line.
5/16 x 1/4 BSPT as above with a 5/16" gas line fitting.