The RAW mentions under "Other Charisma Checks" that Charisma is the ability used to "Find the best person to talk to for news, rumors, and gossip." and "Blend into a crowd to get the sense of key topics of conversation."
But which skill, is the question?
The game poorly handles the idea "use any skill with any ability". People pick skills assuming they will be used with certain abilities. No point in picking skills if you have to use poor ability scores.
So it's unreasonable to "suddenly" ask for a skill check that normally is associated with a different ability. Much better to have a new skill that already is associated with Charisma.
The game isn't set up for the idea where you have few big skills that gets used with different abilities in different situations. The intrinsic value of keeping the number of skills very low just isn't there.
For instance, using Investigate (Charisma) for gather information or gossip checks is irritating if you have a low Int. Why take a skill you normally suck at? You're much better off convincing your DM you can use Persuasion instead.
The entire problem is created by WotC going overboard with skill reduction.
Or is the flat Charisma roll appropriate?
Not unless people become much better at assigning lower DCs for flat ability rolls.
Gather Information is not an inherently more difficult task. Flat ability checks are by design more difficult if the DCs remain constant because you don't add your proficiency bonus to the roll, because there's no skill associated which you can be proficient in.