The PDF User Guides in the GDTk documentation collection are linked here.
The Eilmer User Guide is a good place for getting started with the simulation code.
The Geometry User Guide contains details on the geometric elements available in your Eilmer input scripts for building Eilmer-native grids.
The Gas User Guide inroduces the basic gas modelling capabilities available for Eilmer.
The Reacting Gas Thermochemistry Guide is useful for those wanting to do reacting flow simulations. It contains information on the thermally-perfect gas model and how to construct a finite-rate chemistry input file.