Koos liked the music, and plugged it in the peloton. Apparently others liked the song too. In return, the gasoline bros challenged him with a bet, and promised to write a song for him, if, and only if he would win the national jersey. So Koos did, and that's how this song was born...
If you like their music, you can download their first album, entirely, HERE
On koosmoerenhout.nl the supporter who brought the flag said this:
My name is Niko. I'm a 23 years old French guy. I've always supported Rabo team (for more than 10 years!). Even if I never met Koos, I admire him a lot, as a great road captain, and also certainly a great man. I often read his Twitter's updates, and recently, I've discovered the song "There it goes" by Gasoline Brothers. That's why this is the message I've painted on the flag (the one you saw on TV). I even tried to play the song during the Stage 5 (Tarragona-Vinaros), but I wonder if he really heard it.
Now I'm back in france, Vuelta is over for me, but I wish the best for Koos and the whole Rabo team for the upcoming days.