Tax on crypto transactions

The provision of the crypto loan itself is not subject to GST.

According to IRAS (e-Tax Guide),

2.1 With effect from 1 Jan 2020, supplies of digital payment tokens will no longer be subject to GST. Specifically:
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(ii) A supply of digital payment tokens in exchange for fiat currency or other digital payment tokens, and the provision of any loan, advance or credit of digital payment tokens will be exempt from GST. Therefore, the supply of such tokens, being an exempt supply, will not contribute to your annual taxable turnover for the determination of your liability for GST registration. 
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Loan of digital payment token
6.5 The provision of any loan, advance or credit of digital payment tokens is an exempt supply. Example 7 – Loan of digital payment token scenario:
GST-registered company F makes loans of Digital Payment Token Z in return for interest income. With effect from 1 Jan 2020, E will report the interest received as its exempt supplies in its GST return.