(No, I don't know why it says it's a teamwork feat. It's not)
Thread necro with actual information. This is summed up from the information in 'Hell's Vengeance 3, the Inferno Gate', but I expect this information is probably also to be found in PZO1141, Planar Adventures.
Soulgates are apparently-surprisingly-common links between planes that are hidden in plain sight -- 'naturally formed or exceptionally old archways, doors, or other closed geometric shapes.' Tales of strange feelings, rumors of hauntings, and the like can give hints to the existence of one, but they don't show up to detect magic or similar spells. They usually have the alignment of the plane on the other side, but rarely may (if they were created by or frequently used by being(s) of a specific other alignment) bear an alignment different than that of said plane.
Soulgates are specific point-to-point gateways, (meaning their endpoints do not change) and such connection locations do not appear to be random.
After that, well -- as the feats say.