for under-construction flats booked prior to July 1, 2017, where the payments were made prior to the GST coming into force, the builder would have already recovered the service tax and VAT, applicable on such payments made.

2) for the balance consideration, which remained unpaid and for which the builder has not raised an invoice, the builder will recover the GST at the rate of 12 per cent on the balance amount due.

3) if full payment was made by you in 2015 no GST was payable by you 

4) issue builder legal notice for bogus GST charged . 

5) complain against builder before RERA 

Well, your haste made you financial loss. Further Transfer of completed property is not considered as rendering services to a buyer and thus does not attract GST. GST was never applied on your poverty. You are cajoled by double tax. Vat + GST.

GST applies to payment made after it`s enactment , builder have already recovered the service tax and VAT from you.

File police complaint agasint him of extortion and cheating. Ask him to give you GST receipt.

1. GST is made applicable from 1.7.17

2. prior to GST there was service tax

3. if all payments were made prior to commencement of GST, then the builder had already collected service tax from you for those payments and hence he had no business to charge GST claiming any differential in the rates of service tax and GST

4. Also it appears that the project is not issued any occupation certificate since you state in your query that builder gave you possession of half complete villas

6. though under GST it is the rule that if the building is incomplete and if any payment is made to builder, then GST will apply on that payment, whereas if OC is received for the project then GST becomes inapplicable

7. it appears that the builder cheated you by demanding difference of amount in service tax and GST rates under the pretext that the project is not issued a OC

8. You have two remedies - the builder has indulged in profiteering activity against which you can file an online complaint against him with the National Anti Profiteering Authority in Delhi. It has a website for that. 

9. secondly the act of the builder is an unfair trade practice against which you can either file a consumer complaint or a RERA complaint if the project is registered with RERA authority 

10. a police complaint for cheating with the police's Economic Offences Wing can also be filed. In fact other buyers who have similarly been duped can join you in your complaint. 

You can file a case of cheating and breach of trust against the builder and a consumer complaint to recover the amount illegally charged along with the interest and compensation for the mental harassment.

You can complaint to gst authority for the same and also file consumer complaint against her

file criminal case against builder  u/s 420,468 of ipc. alternative you can file complaint against builder before consumer court. 

How do you say that the builder has cheated you in the name of GST and had taken payment from you on that name?

If you have proofs for that you may first issue a legal notice to him demanding your amount, and you may also state that if he is not returning the amount collected from you in the name of GST then you would approach the concerned authority with a complaint against him for this fraudulent claim.

After that you can decide further legal action as per law against him

Dear

If you have paid full amount along with VAT before GST came into force then you are not liable to pay the GST now for taking possession of villas. 

Now you should send a legal notice to Builder for refund of GST if doesn't refund the money then you should lodge a complaint with GST grievance redressal portal.

Also file a complaint with RERA for charging fake GST from you.