Payment gateways can be configured in GENERAL > PAYMENT GATEWAYS. Each gateway has their own specific API requirements in order to process transactions so they will vary from gateway to gateway. However, all gateways have 3 core options:
SHOW ON ORDER FORM - Checking this box will mean it can be shown on the order form as an option - subject to product group gateway restrictions
VISIBLE NAME - The name that is used for this gateway throughout the system and to clients (eg. gateway can be Authorize.net, visible name might be Credit Card)
CONVERT TO FOR PROCESSING - This option is for when using multiple currencies. It is possible that not every gateway you use will accept all the currencies you offer on your site and so for those that don't, you can choose a currency here for PHPTRAVELS to convert to before sending to them. For example if using Google Checkout with a UK account but offering prices in GBP & USD, you would need to select GBP in the Convert To box. This will use the rates set in Config > Currencies to do the conversion which can be set to auto update daily.
DEACTIVATING GATEWAY MODULES
Should you no longer wish to use a particular payment gateway clicking the red (Deactivate) link beside the name will gracefully remove the gateway from the system. This will prevent any further payments being made using this gateway and automatically reassign any other assigned to that payment method to another option. Upon clicking the deactivate link, a popup window will appear prompting for the selection of the alternative payment gateway to which any currently assigned items should be changed to.