Supply under RCM

No the GTA is not required to pay if the recipient has paid tax under RCM and further the GTA has not charged GST in their Invoice. 

Declare it in B2B invoices in GSTR 1, check the box (Supply attract Reverse Charge), in Item Details, enter taxable value and amount of tax with 0.00 and proceed.

No, enter the taxable value at its own rate whichever it may be but the tax amount in IGST/CGST/SGST as 0.00.