Salem

Basically, here is an image of my fence, and whenever I attempt to build a gate between these two posts (the option does come up) I walk forwards and then stop, as if I'd hit an invisible wall. Thanks in advance for any help, and I'm sorry if this is a noobish question, I looked on the wiki and did as it said (or so I thought)

It is not a noob question. Either a)the cornerpost dont line up or b)they are too close together. What most likely happened is the starting cornerpost was not placed directly in the center of the tile, and skewed the entire wall so when you got back around to where the gate was supposed to be it didnt line up correctly.

Right, so is there anything I can do about it or is it just kinda stuck like that now then?

You can tear down that cornerpost and place it manually so it lines up good with the other one to build the gate. Or, you can just build 2 cornerpost right outside(or inside) and build gate. Couple tips for you:

Shift+mousewheel will rotate the object to line up with the grid squares.
Holding ctrl allows you to place the object out of alignment with the grid.

Have enough phlegm to knock down a few sections of fence? if so, extend the fence over the gap, knockdown the other corner section, and then rebuild it (if there's room) from the two you just extended. Now knock down the two in the middle. You should be able to build a gate now.

If you get the option for building the gate, that probably means that the posts are properly spaced. Looking at your pic, I would suggest you flatten the area more, then try again before ripping anything down.

As above, I think more flattening on each side of the gate. It probably wants you to enter and exit on flat/level land.

Is it possible to build gates beetween 2 corner posts (8 lime 8 granite 15 stones) or they have to be starter corners (15 lime 15 granite 30 stones)?

They just have to be corner posts. You should only build one start post unless you're trying to do something odd with your fence.

I had the same problem a few days before... first you should flat the ground behind the "gate" inside your claim it schould be to high, when i'm looking right.
Next you should not stand to near the Corners, when you click on build gate... place you a few steps in front of the hole then click the corner post and choose gate.

Maybe it helps.