I use the Pro model (unfortunately not in a Gulfstream
) and find them very good to wear.
Not sure whether they are as effective as an ANR headset but then I doubt that would be a problem in a jet. I don't use a customised ear piece and have never seen one advertised for it by the company but then I have never really looked for something like that.
The ear buds that comes with the set works in the same way as the in-ear hearing defenders. That is, you squash the bud then insert it into your ear canal. It then expands forming a block to extraneous noise.
I have never found them uncomfortable to wear although you do need to either clean or replace the ear buds regularly (the buds screw off the speaker part) as they become stiff with ear wax over time making them harder to fit in your ear canal.
The only complaint (minor) I have had was that the covers over the frame that holds the set onto your ears can slide off easily. When this happened to me the company gave me a new set of covers free of charge. But I now ensure that the covers are properly in place when I put the headset on and check when I remove it as this is when they can slide off.