What is the "gateway fee" that I see as one of the charges for credit card processers?

The gateway fee basically is the costs to that company for allowing you access / use of the gateway. Usually this is a different company & there is a lot of upkeep to maintain the gateway. There is usually a month charge & then a per transaction costs. If you are selling products that are under $5, this is where your profits can really be low since the gateway fee is based per transaction & not on a percentage of the sale.

Basically a merchant account consists of a CC processor & a gateway. The gateway allows you access to visanet etc to verify the CC.