Gourdin literally translates to "club" or "cudgel," a heavy stick used as a weapon. However, in French slang, it's also a euphemism for a penis.
In the context of this Serge Gainsbourg song, the line "Ça y est tu as l'gourdin t'es en rut" (That's it, you have the club, you're in heat) clearly uses the slang meaning. It's a provocative and memorable word that captures the song's edgy and irreverent tone, making it interesting for learners to discover this double meaning.
This is Gainsbourg picking a fight with a motorcycle. The whole song is one side of an argument aimed at David, a Harley owner he accuses of stealing the bike and running off to sleep with anyone who'll have him: "Qu'est-ce tu décides, tu vas aux putes?" (So what's it going to be, off to the whores?) Between every accusation the title comes back like a chorus and an insult in the same breath, "Harley David son of a bitch," half French slang and half English, so David and the machine blur into one target.
It ends exactly where this kind of jealousy usually points. "Tu es mort sur le coup c'est bien fait" (You died on the spot, serves you right). Gainsbourg wrote it in 1984 for "Love on the Beat," at his most deliberately vulgar, and the joke only lands because he never lets you decide whether he's horrified or delighted that the bike finally killed him.