Filing/Reporting: You do not need to report sample invoices as taxable outward supplies in your GSTR-1 return because free samples are not considered a "supply" due to the absence of consideration.
ITC Reversal: You must identify the inputs used to make these sample tablets and reverse the corresponding Input Tax Credit in your GSTR-3B return as per Section 17(5)(h). No output tax is payable, and movement should happen via a Delivery Challan.