A gas burner apparatus with pre-mixing comprising mixing means for mixing an air/gas mixture which is intended to be conveyed to the intake section of a fan unit, whose delivery is conveyed to a combustion head of the burner, the mixing means comprising a plurality of ducts, each of which has a region in the form of a Venturi duct with contiguous portions (16a, 16b) which are convergent/divergent, with respect to the direction of flow, and with a restricted duct cross-section defined between the convergent portion and the divergent portion, there being conveyed in each convergent portion an air flow which is intended to be mixed with a gas flow which is introduced into the duct at the restricted cross-section, the air/gas mixture being conveyed through the divergent portion of the duct to the intake section of the fan unit, each duct of the plurality being defined by the engagement of at least a first and a second mutually separate burner region (A,B), along a mutual engagement profile, there being carried on each of the burner regions (A,B) corresponding, separate portions of each of the ducts, at least a first fluid-tight sealing element being provided between the burner regions (A,B) along the engagement profile in order to prevent gas and/or air from being drawn in with respect to the ducts, characterized in that the at least first sealing element is interposed between the burner regions (A,B) at the restricted cross-section of each of the Venturi ducts of the plurality of ducts, - in that the at least first sealing element has through-holes which define the restricted cross-section of the corresponding Venturi duct in at least some of the plurality of ducts, as a result of the engagement of the burner regions (A,B) along the engagement profile, - and in that at least the first sealing element is interchangeable with at least a second sealing element (20', 20") which has corresponding through-holes (21', 21"), whose overall size for the relevant cross-sections is different from the overall size for the cross-sections relating to the corresponding holes of the first sealing element, so as to vary the overall cross-section defined by the plurality of holes of the ducts, with at least the first element being interchanged with at least the second sealing element(20', 20"), and consequently to control the burner in accordance with the thermal power required and/or the flow resistances which exist in the ducts for supplying the air/gas mixture to the burner. - in that the at least first sealing element has through-holes which define the restricted cross-section of the corresponding Venturi duct in at least some of the plurality of ducts, as a result of the engagement of the burner regions (A,B) along the engagement profile, - and in that at least the first sealing element is interchangeable with at least a second sealing element (20', 20") which has corresponding through-holes (21', 21"), whose overall size for the relevant cross-sections is different from the overall size for the cross-sections relating to the corresponding holes of the first sealing element, so as to vary the overall cross-section defined by the plurality of holes of the ducts, with at least the first element being interchanged with at least the second sealing element(20', 20"), and consequently to control the burner in accordance with the thermal power required and/or the flow resistances which exist in the ducts for supplying the air/gas mixture to the burner.