The only reason why I've visited Gainborough is to visit family there; apart from that, the only interesting thing in the town is the beautiful Old Hall. It's Tudor in origin and has been added to over the next 100 years, meaning it is an absolutely storybook half timbered and red brick hall. It's the type of place that makes tourists grab their cameras; it really is stunning.
Inside, the main attraction is the remarkably well preserved interior; there are lots of beautiful fireplaces, carved ceilings and wall panelling, and windows. There are informative displays about the history of the building and the era, but not too much else. However, I think this is a good thing; they have a beautiful building and that's what you're there for, so why mess it up with wax works and silly 'interactive' (usually broken) displays? The staff are all very friendly and well informed. It takes about 1 1/2 hours to see it comfortably and take lots of photos!
There's a small gift shop with the usual crafty / 'national trust' (though it's not an NT property) bits and bobs. They have regular craft fairs and concerts - I can especially recommend the Christmas craft fair and carols events. There are also regular recreations of historical events and plays; they have a very full schedule of events.
The lack of other things to do in Gainsborough itself mean I wouldn't recommend a long trip to see just the Old Hall; however, Lincoln is close by and if you are planning a long day/weekend there, Gainsborough Old Hall is well worth the small detour.