A gather info check isn't opposed by anything other than what the DM decides the difficulty is...it's all up to the DM..."If you want to find out about a specific rumor, or a specific item, or obtain a map, or do something else along those lines, the DC for the check is 15 to 25, or even higher. "
IMC, I set up a gather information list of informational tidbits that can be gathered, with easy secrets having a low DC (10-15) and super hard secrets a very high (outside of character ability) DC (up to 50+). This also accounts for the quality of the information. If I say "super-secret" is a DC20, if the player gets a 20, they will only get the information kernal (the king sleeps around), not the whole story with dates, details, names, addresses, implications, etc.
As the DM, you must decide how well the NPC hid the information and/or how sensitive the information is. I think it's totally arbitrary and up to the DM, not the skill of the NPC directly.