Gravelli needs to you make sure a good friend of his can get into Liberty City and give a speech. He knows Dimitri won't want it to be made public, so you'll need to make sure he gets to City Hall alright.
Grab a car or a taxi and head to meet your contact. Once you meet him, you'll be told to get into the car and stay in convoy all of the way until City Hall. Drive between the two cars, trying not to crash into either of them.
You'll come to a road block where the car will be ambushed. You'll get the speaker to safety in a near-by alley, then you'll need to take out all of the people ambushing you. Take out the guys in the alley way opposite, those on the building to the right, and at the end of the street there, then take cover behind the parked cars and kill the few guys on the roof.
There's a health pack in the alley next to the speaker should you need it. It will respawn eventually, so you could possibly get it halfway through the firefight then again at the end if you wanted.
To The City Hall
Once you've wiped out all of the enemies, you'll need to get the speaker to the City Hall. Grab a car from out in the street and drive towards the City Hall, marked on your GPS by a yellow blip. As you get closer, a bunch of enemies in cars will chase you. You need to lose them much the same way you would if it were the police. You can however just hang around at the ambush site for awhile and the four enemy cars will come to you, then it's a simple case of shooting them all and driving as slowly as you like to the city hall.
- Some body armour will prove quite useful in this mission.
- Stay at the ambush scene and wait for the cars to come, then kill them all.