If I understand you correctly, you're looking to create a repeat grid without padding by default. Unfortunately, there's no way to do that without adjusting the padding at some point in time in your workflow. Our default of 20 is defined because we feel that most people would want that by default (for things like contact lists, cards, etc.). If you have a strong opinion that you want it 0 by default, please do feel free to file it at https://adobexd.uservoice.com/
I understand the 20px for things such as contact lists, images, etc that aren't "tiled images". I think a possible better option would be an option to add a patterned background, rather than having to make that tiled background. This is an option that is available in Dw, Mu, Ps, etc.