Is SelluSeller Promotion GWP item will show in Marketplace cart during check out?

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.