nf_island is not an official map, I don't think anyone of us knows. I used to be in charge of working on NightFire maps and they were terribly, terribly, terribly broken. If I recall correctly, that grate was breakable, therefore you had to shoot it or deal a small amount of damage to it, however, NightFire's entities are different than Goldeneye Source, and if the person who ported that revision of the map did not fix it and change said grate to func_breakable rather than a regular brush, there is most likely no way to open it.
