debts of honor

This is all assuming a stereotypical githyanki, of course, a non-evil githyanki might well express gratitude and go along with adventurers to help. Barring that, here are a few situations I can think of:

- Under Orders: The structured nature of githyanki culture makes them soldiers par excellence, but it often makes them follow orders they don't particularly like themselves. If the adventurers that destroy the structure holding the gish save him in the heat of battle, he may be beholden to them according to githyanki tradition. Ripvanwormer's right that githyanki don't generally give a dead streaker about non-githyanki, but suppose that the fight is with mind flayers? The tradition might say that a githyanki saved during an illithid-hunt by a fellow hunter is bound to save that hunter's life in return. Even though the adventurers aren't githyanki, they were hunting illithids and a law's a law.

- Strange Bedfellows: If the gish needs something from his "rescuers" he may need to suck it up and barter a deal. For instance, suppose a gish is captured and then a party of adventurers rolls into town and destroys the jail. Now the gish is in a situtation because he needs to get back to work in the Astral but might not know how to get back there exactly. He demands that the adventurers show him the way back, and they demand that he agree to some service in exchange. Being Lawful Evil, the gish won't just blow the agreement off when he gets back, but he's anxious to ditch the lousy mortals and return to active duty. He's not above twisting the agreement to his benefit, of course, and suddenly the PCs may find that gish they coerced into providing additional firepower has them on the ropes...

- Magical Compulsion: This walks the border of "mind control," but I think it's a separate issue. Before breaking the gish out, the adventurers might stumble on some belongings which were confiscated, including an arcane object which demands the gish's loyalty. This used to belong to the gish's superior who was captured but killed (created to prevent betrayal, perhaps, or to stop mutiny). Now that the object is in the adventurer's hands, the gish can't physically harm them and is stuck following them around until he can get the object back.

In all of these scenarios, "indebted" does not mean "won over" by any stretch of the imagination. Any githyanki stuck tagging along with some annoying non-githyanki is bound to be upset at the prospect and try to strike out on his own as soon as he can. While they're lawful creatures, githyanki are also evil creatures and are experts at twisting contracts. All of the descriptions of baatezu finding loopholes can be used in this situation as well, and don't underestimate a githyanki's ability to rationalize things in bizarre ways. The Lich Queen is everything and sometimes sacrificing the good of the few to protect the good of the Her is the higher moral ground to a githyanki. Imagine you owed something to a group of people and to pay them back they wanted you to smash up your friend's car. Likely you'd tell them to forget it and consider the deal broken because of the people's awful morals. Now exchange "you" with "a githyanki" and "smash up your friend's car" with "do anything against the Lich Queen's wishes" and you start to get the basis of githyanki morality.

Githyanki are more often chaotic than lawful. They were listed as chaotic evil in 1st and 2nd edition, and appear as "any evil" in 3e. They believe in personal freedom first and foremost (with reverence for the Lich Queen and hatred of githzerai and illithids tying this belief for first place); honor is not generally one of their priorities.