A GIS is a computer system capable of capturing, storing, analyzing, and displaying geographically referenced information; that is, data identified according to location.
GNIS data is available from a number of sources and services.
Yes, GNIS actively seeks names of features that no longer exist. There are more than 100,000 such entries in the database now. To search for them, type the word "(historical)" (along with other name words if desired) in the name field.
Suggested corrections and additions to the data are accepted from any source for review, and upon validation, will be committed to the database.
The USGS Geographic Names Project maintains an active and extensive program to add features not in the database, primarily through partnerships with Federal, State, and local agencies, and with other organizations having relevant data.