Most likely you haven't closed the floor beneath the gallows. If the floor is still open you can't walk out onto it to grab the bar. To close the floor, you will have to learn the secret of the Wahrk Idol.
To do this:
1. Follow the catwalk past the Jungle Island Fire Marble Dome and find the tower housing Gehn's Throne Room.
2. Throw the left-hand lever to raise the throne and then throw the right-hand lever to close the floor of the Wahrk Galllows
3. You should then now able to grab and ride the triangle bar up.
To do this:
1. Follow the catwalk past the Jungle Island Fire Marble Dome and find the tower housing Gehn's Throne Room.
2. Throw the left-hand lever to raise the throne and then throw the right-hand lever to close the floor of the Wahrk Galllows
3. You should then now able to grab and ride the triangle bar up.