Over the years, as I've tried to familiarize myself with more and more culture, in particular more and more genres of music, I can look back at a lot of what I've come to call gateways - in a musical context "gateway albums".

Gateways are the works that either transcend notions of genre completely or works made by gifted artists of one genre, simply visiting another. Looking back at the gateway albums once you've familiarized yourself with a genre is always interesting. More often than not, the old gateways pale once you've internalized the true value system from the genre. The point of the gateway was always to show you the values of the genre, not entirely to adopt them. It's mostly a matter of visiting not being.
But I still treasure the great gateways - they truly provide an invaluable service. I'm never going to wake up in 1968 i Harlem with tickets for the James Brown show later that night at the Apollo. How am I going to figure out what that's all about without a tour guide?
I'll give you some examples, to firm up what we're talking about, and try to separate various types of gateway.
Let's start with the work of geniuses. Miles Davis' Kind of Blue is the quintessential example of an album that simply transcends the genre it comes from. You could say it's a jazz album. It clearly is. But it doesn't really matter, because the playing and music is so fantastic that pretty much anyone with ears can understand that it is. Appreciating Kind Of Blue won't tell you if you'd appreciate jazz music as such. What you're picking up on isn't necessarily "what jazz players do". It's what Davis is capable of. What Bill Evans is capable of. What Cannonball is capable of.

For a white kid from Scandinavia most of Prince's albums are good gateways to the funk gods. Once you've come to appreciate the JB's circa 1970, and revisit Prince, you understand that there are limits to Prince's funk. It's clearly no longer the funky stuff that impresses you, more the 80s synths and drum machines. The liberties Prince takes with the funk to make it palatable to a rock audience kills a lot of the fun. The live performances brings a lot of the funk back, but still - it's just not Sly Stone or James Brown or George Clinton.
Prince is a good example of a classic difference between the gateway artists and the real thing. The gateway artists are always individualists. Modernists, not traditionalists. They are always personally present in front of the music, whereas the truly great genre musicians often give off a vibe of being in service of the genre even though they are of course always really great players in their own right.

My personal introduction to latin music - in particular mambo - was not furnished by gateway extraordinaire Buena Vista Music Club (which might not be a true gateway at all, by the way, but more on that later. I got my intro years earlier through David Byrne's Rei Momo album. I've been listening to that again lately, and it actually holds up pretty well. In front of a backdrop of pitch perfect latin, Byrne makes the music simpler for a westernized pop listener by singing some good songs.

Bob Dylan, and also The Band, have been great gateways to american roots music. Dylan is palatable to the adolescent-in-the-80s-in-scandinavia me, because he adopts the "subjective performer" standards I expect, but the music is clearly informed by all manner of roots music.
On a similar note, The Rolling Stones used to do the same thing for the blues. The Stones you listened to because they're the Rolling Stones! but of course they actually know all of the standard repertoire and their music is fully informed by tradition.
For pure country, a European like me could do a lot worse than listen to a couple of Elvis Costello albums: King of America for the easiest way in and Almost Blue for a more genuine visit to country music.

More recently, Oh Brother Where Are Thou has done a stellar job of playing roots music for a new audience - but that's not a true gateway album in the sense I mean. That's an actual showcase of the real thing - like Buena Vista Social Club mentioned above - made possible by sugaring with moving images. The Commitments soundtrack-album comes closer to gateway-ism for soul music, since it's clearly soul played by an all white tribute band.

Every genre deserves great gateway artists.