Q. Is Suzuki Gixxer SF 250 suitable for long ride and is pillion seat comfortable for long ride around 500-600 kms?

IMO pillion is comfortable for longer rides. But it will be difficult for the rider as pillion weight come very much on the rider while breaking and it can squash nuts.

Depends on the pillion weight as well as the rider weight , both should never cross 200 , as its unsafe not only for the rider but for other people on the road as well.

Yes it is good for pillion. Not as good as dominar's pillion seat but not such horrible as R15 's. Still you can say it is good one for cruising around 150 - 200 km.