Buses from Grodno to Bialystok go about 1 hour 55 minutes, back about 2 hours 5 minutes. Time varies a little. I believe that this is due to the peculiarities of registration on the border. You can get to Grodno only through Białystok. From Gdansk to Bialystok the train runs on schedule. If the road causes inconvenience, you can revise the route and stay.