Depends what you want. There are some pleasant residential areas but a friend who lives there tells me crime rates and anti-social behaviour have increased dramatically over the past year or two.
I grew up and went to school there when it was a thriving market town but now, I wouldn't live there if you paid me.
The old engineering industries have gone so there is little local employment; public transport is inadequate at best so car ownership is more or less essential; apart from the Marshall's Yard retail site, shopping isn't wonderful; the twice weekly market is a shadow of what it used to be.
I last visited 2 weeks ago to meet an old school friend. We both live about 20 miles away and Gainsborough was a "halfway point" for a lunch date. By time we parted, we were in full agreement that we're glad we now live somewhere else. In short, the town is depressed and depressing.
On the plus side, there are pleasant walks to be had; the Riverside area has been opened up and, if the annual Riverside festival is ever resuscitated, it's great fun. You're not far from the Wolds with wide open countryside and Gainsborough Old Hall is worth a visit.
There are some very pleasant villages around the town, many with an active social scene and, if I had to live in that area I would look hard at those rather than the town itself.