Gear transmission unit, particularly for a motor-vehicle, comprising: - a supporting casing, - a plurality of shafts (A1, A2, A3) rotatably supported within the supporting casing by means of respective rolling bearings (B1, B2, B3), - a plurality of toothed wheels (R1, R2, R3, R4) carried by said shafts (A1, A2, A3) and meshing with each other, - a forced lubrication system for lubricating said rolling bearings (B1, B2, B3) and/or further inner components of the transmission unit, said forced lubrication system including: - a supply pump for supplying the lubricant, and - a plurality of channels formed within walls of said supporting casing of the transmission unit, for bringing the lubricant adjacent to the inner components to be lubricated, - at least some of said channels being configured for bringing a forced flow of lubricant up to one or more channel outlets adjacent to one or more components to be lubricated, - said transmission unit being provided with one or more accessory elements in the form of lubricant distribution boxes, - wherein the, or each, distribution box has a hollow body rigidly connected to the supporting casing adjacent to a respective channel outlet which supplies a flow of lubricant under pressure, - wherein said hollow body defines an inner lubricant distribution chamber and has an inlet connected to said respective channel outlet and two or more outlets connected to further conduits or channels or chambers, for supplying a flow of lubricant under pressure in parallel to various components to be lubricated inside the supporting casing, - wherein said distribution box comprises a box element and a lid secured to the box element, and - wherein said distribution box has a substantially flattened body, with two opposite main walls respectively defined by said box element and said lid and in that the main wall of said box element has holes defining the inlet and the outlets of the distribution chamber, - said distribution box being housed within the seat of a rolling bearing of said transmission unit. - a supporting casing, - a plurality of shafts (A1, A2, A3) rotatably supported within the supporting casing by means of respective rolling bearings (B1, B2, B3), - a plurality of toothed wheels (R1, R2, R3, R4) carried by said shafts (A1, A2, A3) and meshing with each other, - a forced lubrication system for lubricating said rolling bearings (B1, B2, B3) and/or further inner components of the transmission unit, said forced lubrication system including: - a supply pump for supplying the lubricant, and - a plurality of channels formed within walls of said supporting casing of the transmission unit, for bringing the lubricant adjacent to the inner components to be lubricated, - at least some of said channels being configured for bringing a forced flow of lubricant up to one or more channel outlets adjacent to one or more components to be lubricated, - said transmission unit being provided with one or more accessory elements in the form of lubricant distribution boxes, - wherein the, or each, distribution box has a hollow body rigidly connected to the supporting casing adjacent to a respective channel outlet which supplies a flow of lubricant under pressure, - wherein said hollow body defines an inner lubricant distribution chamber and has an inlet connected to said respective channel outlet and two or more outlets connected to further conduits or channels or chambers, for supplying a flow of lubricant under pressure in parallel to various components to be lubricated inside the supporting casing, - wherein said distribution box comprises a box element and a lid secured to the box element, and - wherein said distribution box has a substantially flattened body, with two opposite main walls respectively defined by said box element and said lid and in that the main wall of said box element has holes defining the inlet and the outlets of the distribution chamber, - said distribution box being housed within the seat of a rolling bearing of said transmission unit. - a supply pump for supplying the lubricant, and - a plurality of channels formed within walls of said supporting casing of the transmission unit, for bringing the lubricant adjacent to the inner components to be lubricated, - at least some of said channels being configured for bringing a forced flow of lubricant up to one or more channel outlets adjacent to one or more components to be lubricated, - said transmission unit being provided with one or more accessory elements in the form of lubricant distribution boxes, - wherein the, or each, distribution box has a hollow body rigidly connected to the supporting casing adjacent to a respective channel outlet which supplies a flow of lubricant under pressure, - wherein said hollow body defines an inner lubricant distribution chamber and has an inlet connected to said respective channel outlet and two or more outlets connected to further conduits or channels or chambers, for supplying a flow of lubricant under pressure in parallel to various components to be lubricated inside the supporting casing, - wherein said distribution box comprises a box element and a lid secured to the box element, and - wherein said distribution box has a substantially flattened body, with two opposite main walls respectively defined by said box element and said lid and in that the main wall of said box element has holes defining the inlet and the outlets of the distribution chamber, - said distribution box being housed within the seat of a rolling bearing of said transmission unit.