Rushabhdev Bhagwan, who is the first Tirthankar in Jainism, belonged to the Ikshvaku kul (clan) and his gotra was also Ikshvaku. In Jain traditions, it is mentioned that Rushabhdev Bhagwan was born to King Nabhiraja and Queen Marudevi in the Ikshvaku dynasty, which is considered a highly respected royal lineage.

So, the answer is:

Kul/Gotra: Ikshvaku