Dimethylates a single guanine residue at position 26 of most nuclear- and mitochondrial-encoded tRNAs using S-adenosyl-L-methionine as donor of the methyl groups (PubMed:10982862, PubMed:28784718, PubMed:37204604, PubMed:39786990). tRNA guanine(26)-dimethylation is required for redox homeostasis and ensure proper cellular proliferation and oxidative stress survival (PubMed:28784718). {ECO:0000269|PubMed:10982862, ECO:0000269|PubMed:28784718, ECO:0000269|PubMed:37204604, ECO:0000269|PubMed:39786990}. This is the function of TRMT1 (tRNA methyltransferase 1, ENSG00000104907).