Seagull is a slur used to describe Filipinos in a derogatory way. Possibly from how Filipinos often travel overseas to work like migratory birds (although the seagull is not migratory...). Heard in Japan, used by non-Japanese.
The word Seagull is a racial slur that has been used historically to degrade and dehumanize filipinos. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to filipinos. If you are unsure about whether or not it is appropriate to use the word Seagull, it is best to err on the side of caution and avoid using it in any situation.