I'm in chapter 2, aligned with the Old Camp, and have been assigned to spy on the Sect Camp. The Sect Camp leader wants me to go to the Old Mine and kill some more minecrawlers for their secretions, but when I approach the site do I find the portcullis closed. I can use the nearby winch, but the entrance remains shut.
Did I just stumble across an insurmountable bug? If so, is there a no-clip exploit or cheat I can employ to get through it? I'd appreciate any help on accessing the Old Mine so that I can resume the game.
Thank you in advance.

Despite I didn't experienced the bug myself, I heard it is a common one.
I am afraid there is no work-around nor cheat code.
You must restart from an earlier save (hope you do not have a too old one).
As a matter of fact, it is recommended to make a save before using any winch because they are all potentially bugged.

You can get around this easily with Marvin mode.
Getting in doesn't even require cheating; you should be able to target a chunk of ore on the left of the door (it's all but invisible in the darkness, but if you point in its general direction and activate it you should phase through the grate and start the mining animation). If that doesn't work, or when you need to get out, open the character screen with S, type "marvin" (without quotes) and hit S again. "Marvin-Mode" should appear in the top left of the screen. You can now jump through the gate by pressing K. To disable Marvin mode, hit S, type "42" (again without quotes), and hit S again.

Wow, nice homage to the Douglas Adams masterpiece. :-)

I thank everyone for replying so quickly to this post.
I tried targeting the ore that was just past the gate, and that did the trick! The problem's been solved, and I'm quite relieved that I didn't have to revert to an old saved game (since I didn't have any prior to my winch operation).
Good community here.

I have learned something! Thank you for the tip.

when typed marvin, it says clerks in top cornor, it slows game and even when pressed K, i cant get passed gate ...

Yes,i entered,but then i tried to go out,but i couldn't do it....With the S key you go back with your character ,what do you mean about S"?

S is the default keybind for opening the character sheet (this is where cheats are typed in). If you have changed it to something else you should be able to substitute your particular key, although I haven't tried it with a non-default binding myself.

Marvin mode still works, even with a different key-set. Just go to the character screen and type the aforementioned marvin.

Be careful when transporting yourself uphill, as you may easily clip through the floor and either land somewhere else or (if no other part of the level is below you) fall into nothingness.
Another thing to note, is that the k-command does not directly transports you forwards in Gothic 1, it transports you "north" (or the ingame equivalent thereof). That shouldn't be a problem with this particular gate, but it may make that command useless to circumvent other bugs.

Any key you've assigned to statistics. WASD controls use "B" or "C", for example. It works for any keybind (except M, A, R, V, I or N, obviously :P).

You're mistaking K with F8 here. In G1, F8 sends you flying north and K moves you forward. In G2, F8 sends you flying forward and K moves you forward.