Advance payment refund with gst

It's long period. you booked in Dec 2018. supplier may not refund GST now I guess. He can file a refund claim and once he receives refund, he can refund you the gst amount.

with regard to the advance amount, he is ideally liable to refund entire amount unless he deducts any charges towards cancellation.

In connection with covid19 the entire amount must be returned to the concerned person

There is also circular issued by cbic which clarifies this position

It is hold good