Noun. When white people (see stuff white people like) use a single foreign song that has entered mainstream American music, generally through movies, to gain cultural entre into the music of an exotic country/region. By using pandora, related videos feature of youtube, or other tools for finding similar songs the white person is able to build a small, narrow understanding of music from a foreign place. The role of the gateway song is vital in the white person's understanding of a foreign music source because it will be the standard by which all other songs "discovered" in the genre will be judged. However, it is important to realize that the white person will never veer far from the gateway song.
Finally, it must be noted that white people are acutely ashamed of the gateway song because it proves that he/she was completely unaware of a genre of foreign music until exposed by the distinctly unauthentic mainstream media. White people would prefer to be exposed to "world" music from browsing non-chain record stores, study abroad programs, and non-white friends. Bonus would be a combination of a musical "discovery" while falling in love with a foreign person. However, since these more authentic methods of "discovering" "world" music are not always available, gateway songs are a necessary evil for the average white person.
Jai Ho-gateway song to bollywood, anything by Bob Marley-gateway song to reggae, anything by Sean Paul-gateway to reggaeton, 12 girls band version of Clocks etc.
It is a song (a single) that brings people in to listen and causes them to explore the band/genre after listening. After getting into the band/genre, the gateway song does not seem as great anymore. Those kinds of songs are identified with the band and are usually not as great as the other songs that the band has written. Not to be confused with songs that are one-hit wonders.
Guy 1: Welcome Home is an awesome song by Coheed and Cambria.
Guy 2: Dude, that song sucks. It's just a gateway song. They have way better songs.