Here is what is currently known about "The Halloween Surprise" update. The update will include two new cars, a lot of new masks, new audio files, new weapons, and more. Here is the compiled information courtesy of both GTA Forums and Reddit.
Lurcher
[ATTACH=CONFIG]1889[/ATTACH][ATTACH=CONFIG]1890[/ATTACH]
Btype2
[ATTACH=CONFIG]1891[/ATTACH]- Two Cars: One called a Lurcher - looks like a hearse and has a zombie in the back. The other is named Btype2 - looks like a convertible Roosevelt
- Masks: Pumpkins, devil, zombie, serial killer and monster types
- Audio files: Might suggest new jobs/freemode events. Some mention "dog" and "creature", this could suggest animals are coming to Online.
- Weapons: Seems to be a few new weapons including a new gun (submachine gun?), a flashlight and there were a few other but no pictures.
- New T shirt emblems: Basically just Halloween themed logos and phrases roughly about a dozen of them
If the update isn't out tomorrow, then we know to look for it closer to Halloween itself.