In SNPedia, which Promethease reports are based on, there are both single SNPs and combinations of SNPs. Combinations (sets) of SNPs are called genosets, and the abbreviation is "gs", so Gs221 is genoset #221.
To find out more about the publication that says what the consequence of having that combination of SNPs is, either go to the Gs page in SNPedia or click on the /criteria link from that page, which lists the SNP names.
In this case, the cited publication says your risk is based on which two HLA DQ molecules you are carrying (look at Table 1). Gs221 says you have one DQ2.2 but to get a better estimate of autoimmune risk you need to know what your other HLA-DQ allele is.