An evening's time, a few gold pieces for buying drinks and making friends, and a DC 10 Gather Information check get you a general idea of a city's major news items, assuming there are no obvious reasons why the information would be withheld. The higher your check result, the better the information.
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.
See also epic usages of Gather Information.
A typical Gather Information check takes 1d4+1 hours.
Yes, but it takes time for each check. Furthermore, you may draw attention to yourself if you repeatedly pursue a certain type of information.
A half-elf has a +2 racial bonus on Gather Information checks.
If you have the Investigator feat, you get a +2 bonus on Gather Information checks.
If you have 5 or more ranks in Knowledge (local), you get a +2 bonus on Gather Information checks.