Important: Please be aware that support for the OpenType 1.8 specification (and therefore, Variable Fonts) is only preliminary at the moment. Variable Fonts exported in Glyphs 2.4 are not fully compliant to the specification yet. Do not release Variable Fonts generated in Glyphs yet.
The idea is that you can prepare and test your font families (with Google’s fontview app and Apple’s recent Webkit nightly builds), so that, when Glyphs fully supports OT 1.8, you are prepared and can release quickly.