How likely would an open, accidentally swallowed office sized staple be to cause any damage to gi tract?

Go to Emergency Rm: There's no way to know what that staple is going to do, but it needs to be followed. You should go to the er or your doctor for an evaluation. They will likely want to follow it with an x-ray. If you did this, keep the staples out of your mouth. If it's somebody else, like a child, there's no telling how many times they've done it before so there may be quite a few more staples in there already.

Unlikely: These type of foreign bodies tend to pass through the GI tract without much harm done. If, however, there are symptoms of severe pain, vomiting or bleeding, then you should go to the er.