<p>I've recently purchased an Ender 3 and have had great success with some Cura settings found on a YouTube Tutorial at 0.2 mm resolution.</p>
<p>So then I noticed that there were default settings in Cura for the Ender 3. Except printing at 0.2 mm it selects a 20 % infill, and when choosing 0.1 mm it changed the infill to 10 %.</p>
<p>I changed infill to 20 % and attempted to print this but there were gaps in the bottom layer and it won't stick to the bed. Is there anything else I need to change in the process?</p>
<p>The shape is essentially a cube with a circular hole in the middle, sliced in half.</p>

Basically you have 2 issues, first, an adhesion in combination with layer thickness problem, second, an infill problem.
Starting with the infill issue, when you lower the layer height, without increasing the amount of layers for the "Top/Bottom Thickness", you get a very thin shell (unless the top bottom thickness is expressed in mm). A lower layer height should, because of the lesser amount of filament being extruded over the infill, should be accompanied with a higher infill value, but that is necessary for the top layers, your issue is with the bottom layer and adhesion. As said, a lower layer height also implies lower filament flow, for the first layer this lower flow causes an inconsistent flow to adhere the filament to the bed (probably caused by the gap between the nozzle and bed from leveling with a piece of paper). Most slicers will add some extra features to increase the change to get the filament to stick to the bed; one of those is an increased first layer height (e.g. in Ultimaker Cura, the first default layer height for Ultimaker 3 printers is laarger than the rest of the layers), others include modifying the flow by e.g. over-extruding for the first layer.
You could try to increase your first layer height to the value you create successful prints with, specify the thickness of the bottom and top (or increase the amount of layers for printing these) and increase the infill percentage.