No, the gift product will not be shown in the order details during checkout or marketplace invoice since GWP  promotion is a non-integrated promotion with marketplace.

For e.g. if user has create a GWP promotion in SelluSeller Buy product X and get product Y free. So when customer buys product X on platform the order gets sync in SelluSeller and SelluSeller will add product Y in order during promotion period. Customer will not see the free gift product on marketplace order details/invoice.

We hope the details above were helpful for you. Please feel free to share with us if require further assistance and will be happy to take it up.