Hi Will,
I strongly recommend you do not ignore the warnings about invalid genotype fields as they are there to ensure your data is not corrupted!
If you want to ignore the checks Hail places on FORMAT fields, you can use import_vcf with the arguments generic=True and specify the FORMAT fields you want to be treated as a genotype call with call_fields. The GT field is automatically imported as a genotype call.
See this discuss post for more information on working with generic genotype fields.
Best,
Jackie