Oyster and Railcard purchase at Gatwick Airport

Railcards Assuming you actually mean a railcard as in the discount card, according to the how to buy page for railcards (in this case the 16-25, but you can switch to the others from the menu), they can be bought online or by phone with a UK driving license or passport (which I assume you don't have) or at any station with a manned ticket office.

You can see what facilities a station has from this page on the National Rail Enquiries site. According to that, there is a 24 hour(!) ticket office at Gatwick. Be prepared to queue however.

I'd also reccomend checking that a non-UK resident is actually allowed a railcard.

Oyster Card This page on visit london (thanks DumbCoder) specifically mentions Gatwick as a location to buy oyster cards. Transport For London's site however only mentions visitor cards being available through the post and various international agents. It may be safeest to buy one in advance. The £18 you have been quoted is probably £3 for the card itself, and £15 for the minimum balance you can load on to it.

I didn't mean a Rail discount card, I meant a ticket/pass Train tickets can be bought from any station with a ticket office or machine. See the above NRE link to check if a station has these facilities. If you are boarding at a station without a way of buyng tickets, you can board the train, then immediatley seek out a conductor and ask to buy a ticket.

If you want a broader "pass" for Rail travel in the UK, then EU residents should look towards single country passes from Interrail, while non-eligible nations can look at Britrail. (Request for help here - it's not clear to me which of britrail.com and britrail.net are the offical site)

You can purchase vistor oyster card in gatwick airport train station. It cost £3 plus at least £10 credit. You can receive discount fare when using oyster card

Oyster card can be used in:

Tube, DLR, TfL Rail and London Overground:All services in Zones 1-9 and to Watford Junction and Shenfield.

National Rail:All suburban trains stopping in Zones 1-9 and on journeys to:Chafford Hundred, Grays, Ockendon, Purfleet , Watford Junction , Broxbourne

But not on:Heathrow Express, Heathrow Connect services between Hayes & Harlington and Heathrow, Southeastern high speed services between St Pancras International and Stratford International, Any journey starting or finishing outside Zones 1-9 or beyond Watford Junction, Broxbourne, Shenfield or Grays

I dont sure what railcard mean. If you mean rail ticket, you can buy it in ticket machine/counter, or purchase online and get it in ticket machine with your debit/credit card and reserve code. Like flight ticket, purchase early usually costs less.

There are 3 types of rail ticket

Anytime: most expensive but you can travel anytime

Off-peak/super off-peak:less expensive but travel time is limited

Advanced: Fixed traveltime, limited availability, cheapest

If you mean the seperate dicount card which offer 1/3 fare off, you can purchase it online/train station counter. It cost £30 except £20 for disables annual. Most rail card has requirements on cardholder, except network railcard but it covers smaller area. You need a passport photo for some of the rail card and you have to show both railcard and ticket when staff is checking ticket on train.

The fastest way from Gatwick airport to Euston station is going to Victoria station first and interchange Victoria line to Euston station. The trip takes not more than an hour. The train between Gatwick to Victoria station is served by Gatwick Express and Southern, the latter trip is longer but cheaper.

Finally you can search these website for more information.

https://www.tfl.gov.uk/plan-a-journey/

http://www.nationalrail.co.uk

PS1: Gatwick train ticket office opens 24hours, London Victoria ticket office opens 4am-1am except 6am-1am on sunday

PS2: The queue in gatwick ticket office is not long, but in Lonlon Victoria you have to wait 5mins