Bless you! This pronunciation would be spelled "Paliaci" or so in Fiorentino-standard Italian. I take it from your username that you are a southerner? Obviously, there are no "right" answers--it's just the guy who's in charge when they make the rules who gets to be right.

The school-standard pronunciation would be kind of like Poh yee ah ghee (where ghee is like the butter with a hard g)

Ee tahl kor say Mee lahn oh

Gipiemme is easy for Americans, just say the names of the letters as you would in English: G P M A. In fact, isn't this brand a play on the founders initials, GPM? (in Italiano, these letters are pronounced Gee Pee Emmay)