The key differences between Freemode and Deathmatch PvP in GTA Online and how to adapt your playstyle for success in both.
Being a multiplayer title you will encounter other players in Grand Theft Auto Online on a constant basis. Doing any activity that doesn't take place in Freemode guarantees interaction with others, and even in Freemode you'll be bumping into other players time and again.
And since this is GTA we're talking about, the overwhelming majority of player encounters will result in a PvP firefight.
Whether you're fighting in a controlled Deathmatch, or some mook is giving you a hard time in Freemode, honing your PvP skills is essential for victory and survival. Most of GTA Online's systems are combat related, and knowing your way around these mechanics is worth almost as much as raw skill when it comes to aiming — though actually hitting your targets is also essential.
The Basics

We're going to assume you're coming into this knowing the control layout of GTA Online, so we can get down to business.
First of all, PvP encounters in Freemode will be very different to controlled circumstances like in a deathmatch as the latter restricts the tools at your disposal and puts you on even ground with your opponents.
In deathmatch, PvP is more reliant on skill, while in Freemode it's all about who got the jump on who, who has the bigger guns and who is flying the Hydra.
Head or Body?
Knowing your current proficiency in aiming is useful. While practice always helps, if you know you can't reliably flick those headshots in the heat of battle, instead of raking up miss after miss in the hopes of that damage modifier, try shooting at your opponent's body. The increased damage of headshots counts for nothing if you don't actually hit.
Like any other shooter, GTA will be filled with those "we started shooting at the same time but he killed me, not the other way around!" moments. Your shots only matter if they connect, so only start automatically aiming for heads when you've practiced aim.
Weapons
GTA Online features a range of weapons with various stats, however in practice those stats matter a whole lot less than in most other games. Obviously, weapons in different classes will be hugely different (like a shotgun and a sniper rifle), but within the same category they're mostly interchangeable — or, at least they were until Gunrunning brought in Mk II. weapon mods, which give their users significant advantages.
Misc
It should go without saying to keep your snacks and armor filled. Other general tips include being wary of personal vehicles left unattended, getting a feel for the map so wherever you're attacked you'll know where to go for cover, and always assuming that you'll be attacked by other players in any situation. You should always eat snacks to replenish health when you are in cover, as this skips the long animation. This can be sped up even more by binding snacks to your quick action button.
Contacts
Making use of your contacts is a key tactic when it comes to combat. One of the easiest way to get the advantage is to call a mugger on your target from Lamar, or by utilizing one of Lester's many services such as Off the Radar, or Bounty which can win you some allies. Calling in Merryweather mercs is another way to tip the scales.