This inheritance list is sorted roughly, but not completely, alphabetically:

[detail level 1234]

Geo::GDAL	GDAL utility functions and a root class for raster classes
Geo::GDAL::AsyncReader	Enable asynchronous requests
Geo::GDAL::Band	A raster band
Geo::GDAL::ColorTable	A color table from a raster band or a color table, which can be used for a band
Geo::GDAL::Dataset	A set of associated raster bands or vector layer source
Geo::GDAL::Driver	A driver for a specific dataset format
Geo::OGR::Driver	A vector format driver
Geo::GDAL::GCP	A ground control point for georeferencing rasters
Geo::GDAL::MajorObject	An object, which holds meta data
Geo::GDAL::Band	A raster band
Geo::GDAL::Dataset	A set of associated raster bands or vector layer source
Geo::GDAL::Driver	A driver for a specific dataset format
Geo::GNM::Network
Geo::GNM::GenericNetwork
Geo::OGR::Layer	A collection of similar features
Geo::GDAL::RasterAttributeTable	An attribute table in a raster band
Geo::GDAL::Transformer
Geo::GDAL::Extent	A rectangular area in projection coordinates: xmin, ymin, xmax, ymax
Geo::GDAL::GeoTransform	An array of affine transformation coefficients
Geo::GDAL::XML	A simple XML parser
Geo::GNM	Base class for geographical networks in GDAL
Geo::GNM::GenericNetwork
Geo::GNM::Network
Geo::GNM::MajorObject
Geo::OGR	OGR utility functions
Geo::OGR::Feature	A collection of non-spatial and spatial attributes
Geo::OGR::FeatureDefn	The schema of a feature or a layer
Geo::OGR::FieldDefn	A definition of a non-spatial attribute
Geo::OGR::Geometry	Spatial data
Geo::OGR::GeomFieldDefn	A definition of a spatial attribute
Geo::OGR::Layer	A collection of similar features
Geo::OGR::StyleTable
Geo::OGR::DataSource	A vector dataset
Geo::OSR	Base class for projection related classes
Geo::OSR::CoordinateTransformation	An object for transforming from one projection to another
Geo::OSR::SpatialReference	A spatial reference system
our
Geo::GDAL::VSIF	A GDAL virtual file system
Geo::OGR::Driver	A vector format driver