Are you asking about frameless cabinets or face frame cabinets? If the cabinets are face frame, simply use connector ends on the two cabinets that are on either side of the gap. Then delete one and edit the other to widen it as appropriate. If it is frameless cabinets, the technique is similar, but you would want to draw the connector stile. See attached images. You also can use SketchUp native tool to create a filler and make it a CabWriter component using the Make CabWriter Component tool. If you have further questions you can email me at firstname.lastname@example.org and we can set up an online demo with join.me.