Do I need to block margin money when placing GTT orders for buy orders?

There is no need to maintain the margin amount when placing the GTT order since the amount is required only when the trigger price is met. However, it is advisable to maintain sufficient cash balance in your account as your trigger price may be met any day. Your GTTs may be canceled at the sole discretion of Zerodha & the Zerodha RMS due to the shortage of cash in your account.