This is a Brodie Gang mission.
"You just heard The Irishman is still alive. Tam Brodie, the dirty Irish bastard, you worked with him for years, but that didn't stop him from framing you to cover his tracks. That crazy fucker has a death wish, I guess it's up to you to make that wish come true. Word is that he's running a gas racket. Blow up a couple of his gas stations. That'll let Tam know you're looking for him."
You need to blow up two gas stations for this one. Just pull up and put a bullet into one of the pumps to set off the whole gas station.
It doesn’t matter what order you do them in, since it’s basically the same either way. Drive up to the gas station and swing wide. The two gangsters will open fire on you immediately. The problem is that the police also show up quite quickly.
The best way to handle this one is to be far away from the gas station with it in view and just shoot it with a rifle killing the guards and destroying the gas station then go to the next one doing the same. Drive the short distance to the safe zone to end the mission.