In the GUI wallet, under the "Send" tab, does the sliding bar labelled "Privacy" refer to the level of mixin? If so, what level of mixing is provided by the "Low", Medium" and "High" options?
Yes, the "Privacy" sliding bar allows you to choose the level of mixin.
Mixin levels are: 4,5,6,7,8,9,10,11,12,13,14,15,20,25
The GUI wallet presets represent the following mixin values: Low = 4, Medium = 8, High = 25.
At the time of writing, the minimum enforced mixin is 2, but the GUI wallet uses 4 as its minimum.