Try using the "per model setting" feature.
- add a support block on the bottom layer, size it greater than your bottom layer dimension (z height should equal bottom layer height). Make sure the block begins at z=0mm
- Click on the support block again (make sure it highlighted) and select "per model settings" on the left tab. And change the Mesh Type to "modify settings for overlaps".
- Select Settings: Surface Mode - change this back to "on" or "normal" depending which CURA version you have.
- I found that the support block mesh type switched back to "normal mode" which was weird. SO make sure you switch it back to "modify settings for overlaps".
- the regular model should be as mentioned with "surface mode" turned OFF & "smooth spiralized contours" OFF
- Slice!
This worked for me, it fixed my strange gap and also the bottom layer issue! :)