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Well it depends upon what the gateway is. osC has a variety of ready made modules to work with several gateway providers. It's simply a matter of downloading, installing and then tweaking the module and its settings to complete.
As for which Processor to use with osC - I suggest you take a spin over to the osC forums [forums.oscommerce.com] and do a bit of searching. The default payment options (there are several) includes Authorize.net.
Better? Define better? osC is very robust and offers tons of flexibility. As do many other carts. And as with most other carts - you need to have the ability to customize it for what you want. Straight out-of-the-box it's not as good as some but there are trade-offs no matter what product you choose to work with.