Refunds under GST are allowed for exports, zero-rated supplies, and when input tax is greater than output tax. Refunds are not allowed for nil-rated, exempted, or normal supplies. The time period for applying for a refund is 2 years from the date of export or the last date of the filing period the refund relates to. For refunds up to Rs. 5 lakh, only a self-declaration is required, while for larger refunds documents showing tax payment are needed. The refund will be granted within 60 days of filing a valid application.