I´ve made a fresh install fo the GOG version; after that, i´ve installed ONLY the Widescreen mod.
Setting the exact resolution I want (1280x720) in the GemRB.cfg of the game, now I have the game area displayed in the desire resolution
However, the menu screen and videos are displayed in the top left hand corner of the screen, and the fonts are reeeeeeeally little, making the dialogues unreadables. The only solution I´ve found for this is, of course, the Ghostdog UI mod. Being this mod not compatible with GemRB, is there any way to fix those collateral damages?