I figured out this trick around 1998, when I was playing wit' GTA for a time. I saw a lot of enclosed areas, especially at San Andreas, where I would love to enter. Although I could never open those huge double doors, I found out a trick, which lets you climb ALL fences, and ALL gates (except the city boundary fence of course). Here'z how y'do it:
- Grab a big vehicle, like a truck, a van, or a tank, where your character slides underneath the vehicle instead of jumping over it.
- Drive the vehicle to the fence y'wish to cross over. The vehicle's front must be EXACTLY where the fence is.
- Exit the vehicle, go to the back of the vehicle, and slide underneath it, while keeping straight. Your character will slide UNDER the fence or the gate, and enter the area beyond.
NOTE: This trick will NOT work vice-versa! So if you have entered an area, which has no exit, you're f*ckED, and you have to kill yourself.