The main points are the GT is smooth bodied white over silver instead of dimpled white, with alloy wheels instead of steel. It often came with an uprated 1200kg axle instead of 1000kg. It has a nicer interior re fabrics and woodwork. It should also have a battery.
Because Eribas have always been expensive the main dealer at the time put together a "GT" spec package of what UK buyers would probably want. As such this is the more normal spec in the UK. Vans that are not GT spec over this period are probably imports from the continent.
Best thing to do is look at these websites of to familiarise yourself https://www.automotiveleisure.co.uk/ and https://www.domfox.co.uk/

Colin

Don't forget the age of the van in the original posting Steve. Things changed when our newer vans, post 2010, came in that complicate matters further.

Colin

Ian,
whilst looking at pictures and better still vans in the flesh, make a list of must haves and desirables to help you narrow things down. It would be very unusual to get exactly the van you want, so compromises will need to be made.
Once Eriba's reach a certain age then the price becomes more condition based than year. A van a couple of years older may appear expensive, but if it has been well looked after it may be a better buy.
You are obviously aware of the dealers in used vans, so I'm assuming you are also aware of:-
https://www.gumtree.com/search?search_category=all&q=eriba
https://www.preloved.co.uk/search?keyword=eriba
https://www.caravansforsale.co.uk/view-all?sstr=eriba
This is the time of year when things pick up, prices rise and good vans at the correct price will sell quickly. You need to do as much research as possible before going to look at a van so you can make an informed decision quickly. If you are in no rush to buy then prices will go down later summer/autumn, but so will the availability. It would also be a good idea to let forum members know where you are in the country as you would hopefully be able to have a route around a couple of vans and have the chance to ask all of those daft questions.

Best of luck - Colin