How can I check whether GST caused part of the difference in my INR settlement?

Check the fee breakdown for that payment rather than comparing the bank credit with the gross converted amount alone. Skydo's homepage example shows GST at 18% of the transaction fee as a separate deduction. It does not show that tax as 18% of the full payment value.

Match the fee and tax figures on the transaction record or service invoice with the amount settled. If the taxable base or amount appears wrong, send the payment reference and document to support@skydo.com and ask for an explanation or correction. This is a check of Skydo's transaction charge, not advice on the GST treatment of your export invoice.