Genes starting with 'Gm-' or ending with 'Rik' are annotated genes that do not have a canonical name (yet).
Based on Ensembl 'biotype' classifications in mouse, genes starting with 'Gm-' or ending with 'Rik' include protein-coding RNAs, long non-coding RNAs, and antisense transcripts.
Interestingly, genes with the 'Gm' prefix seem to be enriched for pseudogenes. Be mindful of abundance estimates from these transcripts, as they could be the result of mis-mapped reads.
Cheers, Mo Heydarian