No, COVID Test is not required in domestic flights. Thermal screening will be done for all passengers upon arrival. Domestic Passengers: 07 days of Home Quarantine.
Best season to visit Gokarna would definitely be from October to March. You will find each activity operational and could enjoy delicious food at beach shacks.
In my opinion .. its all the year round. I had visited in peak summer( you can check my posts) , it was still pleasant. The best would be winters, but thats the time when it would be the most crowded. If you want a budget friendly experience, go for slightly off beat times, but do avoid moonsoons, because it would mean you cant enjoy the beaches, well, then what would be the point of visiting Goa then!