Cortez's right hand man, Gonzales, gave away vital information about Tommy's deal to someone, and that someone organized the ambush. So Cortez wants you to silence Gonzales's loose tongue, for good!
Cortez will provide you with a chainsaw and Tommy will have to use it to hack Gonzales to pieces. Take Cortez's Cheetah parked right next to the yacht and drive to Gonzales' penthouse behind the Malibu Club, then enter. Gonzales will be there with two guards, Gonzales will run away and head into the street, he will run towards the Malibu Club, you will have to run after him and when you reach him, finish him with the chainsaw.
You will get a two star wanted level;kill Gonzales' goons who attack you and take your car. You now have to lose the wanted stars, either get the nearby police bribes or go to the closest Pay 'n' Spray. Once you have lost your wanted level, it's mission accomplished.
HINT: To kill Gonzales faster, use the sub-machine gun instead of going after him (because the chainsaw won't let you run faster). Shoot him first, then finish off the guards. Get in the car and get to the nearest Pay 'N' Spray. You have completed the mission afterwards.
(Colonel's yacht, Tommy Vercetti and Colonel Cortez.)
Colonel Cortez: Mr. Vercetti.
Tommy Vercetti: Colonel.
Cortez: Thank you for coming. Please sit, Lobster?
Tommy: No - Thanks.
Cortez: I'm ashamed to admit that one of the causes of our mutual problems appears to have been the loose tongue of a man I used to trust. I've been carrying Gonzales for years, but now his incompetence reaches new heights! It's only right that you kill Gonzales.
Tommy: Did he do it? It's the money that's important to me.
Cortez: For this kindness I'll reward you, and then we will find your money together. He will be at his penthouse, half drunk probably. Use this...
(Gonzales' penthouse, Tommy Vercetti and Gonzales)
Gonzales: If he dies, I take the money...
Tommy: I'm going to shut that big mouth of yours!
Gonzales: Eh! He's got a blade!
Tommy: Stop running you fat slimeball!
(The street outside Gonzales' penthouse, Tommy and Gonzales)
Gonzales: Oh, sweet Jesus. I've wasted my life and my looks!
Tommy: Stand still and I'll make it quick!
Gonzales: I pay you double. Tommy. DOUBLE!
Tommy: Quit, your squealing, no one cares fatso.