Write a function that takes a gene expression matrix, a vector of gene names, and a vector of sample names, and returns both the subsetted matrix and a vector of average expression values for each selected gene across the selected samples.

• Subset the input matrix to include only the specified genes (rows) and samples (columns).
• Calculate the average expression value for each selected gene across the selected samples.
• Return a list containing the subsetted matrix and the vector of average expression values.