Exx / Fxx decrease / increase the pitch by xx * 1/16th of a semitone on every tick but the first of the row. For EFx / FFx, it's x * 1/16th of a semitone only on the first tick of the row, and for EEx / FEx it's x * 1/64th of a semitone on the first tick of the time. You'll have to look up the units used by the GBA hardware, then you can calculate the equivalent OpenMPT commands.