In the gates entry, one of the questions asked is why the gates lead directly to their locations at first, and then later to the End of Time (with the strange exception of the Tyranus Lair gate).
The theory that's currently there is all and good, but I think Gaspar himself presents an even more basic explanation. He says:
"When 4 or more beings stop into a time warp, the Conservation of Time
theorem states that they will turn up... at the space-time coordinates of
least resistances. Here. Disturbances in the space-time continuum have
increased recently. Far too many folks are just popping in here... I fear
something is having a powerful effect on the very fabric of time..."
That at least explains how they first end up at the End of Time. For why they keep returning, I think this is because they have left friends at the End of Time. Their connection with their friends draws them back to that place each time they warp.
It should be noted that the gate at the Tyranus Lair was directly caused by Lavos, just as some other entity created the gate in Fiona's forest. These gates are special, and don't follow the normal rules. It has its own power, an overwhelming desire to draw the party to the next point in time in which Lavos was activated. Once satisfied, it reverts to the normal functions of any gate.