I've seen that problem before. You could try the following:
1) Download the save editor tool Black Chocobo and then open the FF7 Launcher. Modify the save file so that you're on the next field screen (the one with the base of the pillar where Biggs is hanging over the railing). It's important if playing with the Steam game that the launcher is open when you do this, to avoid the save verification crap that Square put in. If a save file is edited while the launcher is not open, it'll delete the save file altogether the next time the Launcher is started. If that field screen doesn't work out, then try placing yourself on the top of the pillar; that should be a little more stable.
Note, if you're playing the 1998 version and/or there is no FF7 Launcher then there won't be any save verification so don't worry about it.
2) Download the field editor tool Makou Reactor and open the game's flevel.lgp, located in the FF7/Data/Field folder. Make sure to set Makou to run as admin, write-permissions, etc. (if you cannot save your changes to the flevel, try dragging the flevel out onto the desktop, editing it there, then putting it back into the folder).
When you have opened the flevel in Makou, go to field 156 (called mds7plr1). You'll see a listing of groups, etc. Click on No. 4 'Stagedr' and then click on S0 - Main from the small list on the right. This should display the script 'Deactivate the triangle #260'. Double-click on that and change the boolean '1' to a '0'. The command should change when you click okay, and now read 'activate triangle '260'; this is the walkmesh that prevents entry through the gate.
But we're not done just yet; there's NPCs in the way too. Go to Group 19 (man2) and select his Script 3. Select his 'activate contact with the field model' and change that boolean to a '1'; this will deactivate his contact script and let you walk through him. You can now walk through this guy + the gate to reach the other side. Hopefully.
