Güey is a very common and versatile slang term in Mexican Spanish, often used to mean "dude" or "mate." However, it can also carry the connotation of "idiot" or "fool," depending on the context and tone.
In this song, the artist uses it to describe himself as being a bit naive or foolish in love, saying "Yo todo güey, siempre la traté como princesita" (I, all foolish, always treated her like a little princess). It's an interesting word because it's so colloquial and reveals a specific cultural way of expressing self-deprecation in a relatable, almost endearing way.
This song is a fun, slightly bitter take on a breakup where the singer feels he gave his ex everything, and she didn't appreciate it! He talks about treating her like a princess, buying her "ramo buchón" [fancy bouquet], Louis Vuitton bags, and making sure her nails were always perfect. He brags, "Como yo nadie la quería" [Nobody loved her like I did].
But then he realizes she left him for someone less luxurious. He puts it bluntly: "Yo soy un Rolex, pero el Casio le gustó" [I'm a Rolex, but she liked the Casio]. He points out that with him, she had "ropa cara, tenía su clóset lleno" [expensive clothes, her closet was full], and now she's going to discount stores. He even jokes that she probably never saw Paris because her new guy doesn't even invite her out to the club! Ouch! It's clear he's moved on too, saying "en el BM hay una baby, no hay espacio pa' extrañarla" [in the BMW there's a baby, no room to miss her].