I have spent hours working out why changes to my gsl resulted in the start graphic just swirling around and no airbases showing if I then started in the air.
Lesson 1: Check if the CFS3 Terrain SDK\GlobalLayer contents are correct; I must have inadvertently done something to one of the files, as the issue in this case was that the gsl.lib file was not being made correctly! I copied the whole CFS3 Terrain SDK\GlobalLayer from elsewhere and hey presto.