When loading attached savegame, created by the original BG1:TOSC, the time of day and/or the weather rendered by GemRB is different from what the original shows. In addition GemRB plays music, which the original does not. (In the original all I hear is the weather ambient sounds: Wind and rain.)
Savegame demonstrating the problem
Something similar can be observed when loading 888888888-Mission-Pack-Save: GemRB renders no rain, but the original game does.