One coat gloss

One coat gloss is a diy product designed to fool OAP's in B&Q on wednesdays and for albanian van drivers to paint with on their weekend jobs because they think it will be quicker.
Just paint with an oil based under coat until the workpiece is perfectly white. Then over coat in oil based gloss. This is much quicker (sometimes days quicker) than one coat products.
It's just chemistry man. Accept it.

 

Is one coat gloss any good when painting over old
paint or is it best to use an undercoat.

It doesn't matter how old the paint is.
What matters is what condition it's in and what type of paint it is.

Sounds like you'd be better undercoating it