I can't comment on Gestair but can tell you a little about the other two.
Netjets. More regimented, if you don't mind being just a number and one of many they'll suit you. Work 18 days a month, salaries set in stone, more likely to find yourself on minimum rest tours although things have eased off recently. Gateway system means that you can be based within one hour (ish) of an agreed airport from which to start tours. Lots and lots written about them in here, search will tell all.
TAG. A much more relaxed, amiable atmosphere. Salaries, terms and conditions more flexible as they are often negotiable with individual owners. Contracts vary from month on, month off to week on, week off. Some contracts offshore. The company often allow the crew to set their own roster.
Lots of other differences but there's a start for you.