The ornamental design for a built-in gas rangetop, as shown and described.
1. Built-in gas rangetop
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Top
1.6 : Bottom
1.7 : Reference view
The broken lines depict portions of the built-in gas rangetop that form no part of the claimed design; the dash-dot broken lines define the boundary of the claimed design and form no part thereof; reproduction 1.1 is a front perspective view of the built-in gas rangetop; reproduction 1.2 is a front view thereof; reproduction 1.3 is a back view thereof; reproduction 1.4 is a left view thereof where the right view is a mirror image; reproduction 1.5 is a top view thereof; reproduction 1.6 is a bottom view thereof; and reproduction 1.7 is a reference view in which the claimed design is shown in its environment, the environment not forming a part of the claimed design.