If no consideration is involved, then the transaction can constitute a supply, but it will not be considered a taxable supply under the CGST Act, 2017. Consideration is sine qua non to levy GST.
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There are always exceptions …
Consideration is an essential element of supply under Section 7 of the CGST Act, 2017. However, Schedule I acts an exception and enlists various transactions that will be considered as supply of goods or services even without a consideration.