SEOUL, March 5 (Reuters) - Almost 2,500 workers at General Motors' South Korean unit, equivalent to 15 percent of its staff, have applied for a redundancy package that the U.S. automaker is offering as part of a drastic restructuring, union officials said.

GM shocked South Korea last month when it said it was closing down one plant and would decide on the fate of three others in the coming weeks - decisions that hang on potential financial support from Seoul and the amount of concessions it can gain from unions.

At the Gunsan factory which is due to be shut down, 941 out of some 2,000 workers applied for the redundancy package, the officials said, declining to be identified as the information has not been publicly released.

A GM document seen by Reuters showed that over the longer-term, the U.S. automaker aims to cut 5,000 jobs but keep production steady if Seoul agrees to its $2.8 billion proposal for the loss-making operation.

(Reporting by Hyunjoo Jin; Editing by Edwina Gibbs)