I'd like to import the GENCODE Genes GFF3 file with its gene symbols. By using columns on the TxDb object, it is apparent that only the gene_id field is imported, which has entries such as ENSG00000000003.14.How can I also import the gene_name column, which has values like TSPAN6?

The decision was made to not include a gene_name column in the TxDbs. This is explained on the ?transcripts man page:

    Finally, \code{use.names=TRUE} cannot be used when grouping
    by gene \code{by="gene"}. This is because, unlike for the
    other features, the gene ids are external ids (e.g. Entrez
    Gene or Ensembl ids) so the db doesn't have a \code{"gene_name"}
    column for storing alternate gene names.

You can convert from Entrez or Ensembl ids to gene name with an OrgDb package:

> columns(org.Hs.eg.db)
 [1] "ACCNUM"       "ALIAS"        "ENSEMBL"      "ENSEMBLPROT"  "ENSEMBLTRANS"
 [6] "ENTREZID"     "ENZYME"       "EVIDENCE"     "EVIDENCEALL"  "GENENAME"    
[11] "GO"           "GOALL"        "IPI"          "MAP"          "OMIM"        
[16] "ONTOLOGY"     "ONTOLOGYALL"  "PATH"         "PFAM"         "PMID"        
[21] "PROSITE"      "REFSEQ"       "SYMBOL"       "UCSCKG"       "UNIGENE"     
[26] "UNIPROT" 

Valerie