Do not worry unduly. While knowing the Gotra is traditional and helpful for the sankalpa (vow), its absence does not prevent you from performing Shraddh in Gaya. Our rituals are compassionate. If the Gotra is unknown, the Pandit performing your Shraddh in Gaya can use specific mantras or substitute phrases indicating an unknown Gotra, focusing instead on the names and relationship. Lord Vishnu and the Pitrs understand the intention. The sincerity (shraddha) with which you perform the rites in Gaya matters most. Just provide as much information as you know (names, relation, approximate time of death if possible).