Hello,
Someone sent me an email today in regards to this, and I have no idea what that term means.

Can someone please tell me what a gateway advertiser is?

* they were not referring to the company Gateway.

While I'm not 100% on this, I think the term means an advertiser who is trying something for the first time. If successful, these advertisers could open the "gateway" (by proving the concept or medium) for more to come in the future. For example, I beleive ATT was the first company to advertise using banner ads back in 1994. Thus they were gateway advertisers. Their success opened the doors to an entirely new medium.

Could be wrong, but that would be my best guess.