I found that most of the Bootstrap grids for Sketch did not use Sketch’s Show Layout (Ctrl–L) or don’t mark the midpoint of column gutters, so that’s why I made this grid template. I hope you like it!
Please keep in mind that Bootstrap’s column widths are defined as percentages. Rounding to whole pixels can lead to uneven columns, so the grid is occasionally off by a pixel. It should be close enough though for mocking up your designs.