On the bulkhead end of the pin, there's like a cranked lever with a ball stud on the end. One of the gear change cables attaches to this stud. It's a "pop on" type fitting ie. the fitting on the end of the cable is a socket that pops onto the stud - to pop it off, gently lever with a large screwdriver and it will pop off.
Now to get the pin out... mine was the same, very stuck. Some gently tapping backwards towards the bulkhead will persuade it out.
Once out, check it's clean and not rusty - if so, gently clean it up.
The root problem is likely to be the nylon "top hat" bushes that fit in either end of the tube that the pin goes through. These wear and break up.
Push them out (just using your finger usually) and get two new ones from your local Alfa dealer. Now, depending on your model, they might not be listed. I had this problem with my 2.4 JTD. However after a bit of research, turns out they are listed under a 147 TS (same part).
Pop new bushes in, lightly grease the pin and refit, and finally do up nut and pop gear change cable back on.
See this thread from when I did mine: