Mr. Krabs opened it after Al Priss banned Krabby Patties. Later, Miss Gristlepuss found out about the restaurant after Plankton called the cops. After the Krusty Krab got shut down by Al Priss, The Secret Krusty Krab serves as a "backup" to the normal Krusty Krab, after it got closed.
The Galley Grub was not seen in the Secret Krusty Krab. A probable explanation was that Mr. Krabs did not have time to write another one after his original one was still locked in the Krusty Krab.
The Secret Krusty Krab most likely made a rule in which there is a limit to the number of customers present within the building. This probably due to fire code or the building of Secret Krusty Krab isn't as wide as the regular Krusty Krab.