If your car is stolen or written off, and you are forced to file an insurance claim, the insurer would only pay out against the value of the car at the time the damage was done, as opposed to the price you paid for the vehicle.
GAP insurance protects against this shortfall, covering the gap between the insurance payout and the value of the vehicle at the time you bought the car insurance policy.
Some insurance companies will cover the replacement cost of a brand new car in the first year as a standard part of their policy, but I think you mentioned earlier that you're with Diamond and they don't. So you would need a GAP policy to cover you for 3 years.
GAP is a must-have on a new car, really. Annoying that you have to, in effect, pay for insurance twice, but it's a case of better be safe than sorry. Don't accept it from the dealer (who will charge you £400+), shop around online for GAP policies, you could probably get one for about £200.
You will need to pay for a service each year and it will cost more than £100. For a new car, you really need to get it serviced at a dealer so that your warranty is protected. I would work on the assumption that the first service would cost around £150+, the second £250+ and £300+ for the third. They may well not cost you quite that much, but they can and it's always better to overestimate rather than underestimate.
Ask the Fiat dealer about their service packages. You can spread the cost of the services across the 3 years with your car payments, so all you have to worry about is insurance, tax for years 2 & 3 and fuel.