Styling polygon layer as dissolved with QGIS Geometry generator?

You can now do this with the geometry generator in QGIS 2.99/3.0 by using buffer(collect($geometry),0) in the expression builder.

The new collect() aggregate function collects all the feature geometries into a multipolygon; buffer() converts it into a single polygon to be styled as such.

I've tried using the Geometry Generator with no success, and decided on a Virtual Layer and a SQL query

SELECT st_union(geometry) from layer_name

You need to replace "layer_name" with your layer name.

The resulting virtual layer is based on the source layer and will not create any new data.

It's not the geometry generator style, but it should fix your problem.