Yes, iron gall ink is not shelf-stable. There are two ways it can go bad. One way is 'mothering', which happens when the ink generates a gelatinous mass that congeals at the bottom of the bottle. The other way is by precipitating, which happens when the ink generates a grainy sludge that collects at the bottom of the bottle.
I have been told, but have not confirmed through my own experience, that when your ink 'mothers' (gelatinous mass), you can revive it by adding distilled water, and it will turn liquid again. However, when it precipitates (grainy sludge), it's done.
If your ink bottle is plastic, it may have breathed a bit, and allowed ink to escape through the bottle itself, no matter how tight the cap was on.