If you are adjusting the gaps manually, you need to do it for both extension lines. After you’ve got the first dimension set up the way you want it, get the Dimension tool again, type S and click on the dimension. Then go ahead placing more dimensions. You do the same thing for setting arrowhead styles, dimension font and size, etc.
As for not seeing the settings for the fgaps in the Dimensions panel, make sure you are running the latest version of SketchUp/LayOut 2019.
If these gaps are what you want for all project done in your template, make these settings in a new file and save the file as your template. You can delete the dimensions from the paper space before saving but the dimension settings will stick.
