Gothic is one of the best RPGs ever, and it laid the foundations for other great titles to come. It's open-world and mostly non-linear, though completing certain parts of the main storyline progesses your game to the next "chapter" which can change things pretty significantly.
Save frequently and don't skip the dialogue quests, because you're going to want that experience to actually survive outside of the camps (and sometimes inside too). You're pretty much dogmeat early on, and fighting multiple enemies at the start is almost guaranteed suicide. Yes, the game is difficult in the early stages and that's why I like it - you're a nobody that gets beaten up from outset, continually threatened and attacked, but you rise up from that to become a hero.
You can choose whatever playstyle you wish by advancing certain stats and using the proper gear/items. The main builds are melee fighter or mage (archery exists but doesn't seem entirely viable for several reasons), but you can hybrid as you want and there are some thief/rogue skills available as well.
Nearly all dialogue is voice-acted, which was a pretty big deal back when the game came out. A number of NPCs roam around in the camps, making background conversation with each other, wandering off to pee in the alley, and heading to their homes at night, adding a realistic atmosphere to the world.
While Gothic is an amazing game, it's not without its flaws. Enemy AI can sometimes be abused to allow you to casually pick off enemies. The game tends to crash a lot later on (it often has to do with it not loading sound files for the orc weapons properly on startup, which could be a huge problem if you or an NPC/mob has one near your save location). The game lacks a crafting system which was disappointing when a conversation early on with a blacksmith suggests you can actually craft more than a standard sword (you can't). The tools are in place for it, because you can cook meat in a pan and smith some basic items... but that's about it. Additionally, some content was stripped out of the North American version - a side questline involving a bandit camp as well as an in-game music concert (though mods have restored these). There are a number of mods available for the game that range from additional content to crafting to entirely different gameplay styles.
Some experience problems getting the game to run on modern systems, but there are a number of resources/fixes posted on the forums that have worked for most people. That said, it took me about 7 steps to finally get the game to run (and in doing so it wouldn't completely log my full hours played on Steam, which is nearly double what's officially posted). Because of this, I'd recommend buying Gothic when it's on a big sale just in case you can't seem to get it working... but if you do, you'll find a satisfying RPG worth many hours of play.