Hi and welcome Sara,
I've never had any dealings with gumtree personally, but I have read that guys do use it to find ladies to spend time with in exchange for cash, so he could well be genuine.
I would recommend firstly that he not take up too much of your time in email or phone conversations. That tells me he's timewaster and a fantasist. If he hasn't, that's fine.
If you do make arrangements to meet him, make it a short meeting first. Let it be somewhere neutral with lots of people traffic. I would make it lunch, not dinner, so he has no expectations of what might happen afterwards. And I would make it clear that it's just a get-to-know-you lunch. I don't know whether or not you feel you should be paid for lunch.
If all goes well and you click with him and would like to carry things on further, then proceed with the arrangment. Personally, if it's such a large amount of cash, I would take a large chunk of it upfront. I wouldn't recommend a bank transfer as it would require him to have your name, account no. and sort code. You could take a cheque with the top space blank for you to fill in your own name. And don't go through with the booking until it's cleared. Or you could set up a Paypal account on-line and have the funds transferred into that. Or you could get him to fill-up one of those PAYG credit cards with the agreed amount. You have a few choices that don't require him to have your full name if you don't want to give it to him.
Also from the beginning, set your boundaries and have the both you make it clear what you expect from each other so you both know where you stand. There's nothing worse than starting an arrangement with somebody having higher/different expectations than the other one.
I can't think of anything else at the moment, but I hope that helps.