Row 0 = set names, row 1 = descriptions, rows 2+ = items column-aligned.
Examples
tmp <- tempfile(fileext = ".gmx")
writeLines(c("SetA\tSetB",
"desc_A\tdesc_B",
"GENE1\tGENE2",
"GENE2\tGENE3"), tmp)
ds <- load_gmx(tmp)
length(ds@items)
#> [1] 2