Grimmauld Place is a Muggle street in London on which number twelve, Grimmauld Place is found (OP3). Since number twelve is hidden from Muggle eyes, the residents of the street have “long since accepted the amusing mistake in the numbering that had caused number eleven to sit beside number thirteen” (DH12). The street is part of a London square which has the look of being somewhat neglected:
“The grimy fronts of the surrounding houses were not welcoming; some of them had broken windows, glimmering dully in the light from the streetlamps, paint was peeling from many of the doors, and heaps of rubbish lay outside several sets of front steps….” (OP3)
The name may be a play on "grim old place", which as how it appears to Harry when he first sees it. -ibid
We know the street is within about a mile of King's Cross Station (OP10) and of St. Mungo's Hospital (OP23), taking into account the walking times given in the books between those places. Within a one-mile circle around King's Cross there are only a few squares which might qualify to be Grimmauld Place, and most of those have houses far smaller than the description of Sirius' family home. We do know that the nearest Tube station to go to Central London is a small, unkempt station rather than King's Cross, which does limit our search somewhat.
Only one, Lonsdale Square, consists of houses which are tall enough, although in 2007 it was certainly not neglected and run-down. The market value of number 12 in 2017 was over 4 million pounds.
The nearest Underground station is Highbury and Islington, which is somewhat old and decrepit.
Of course, if we're willing to consider that the house may be larger on the inside than outside, any number of less-affluent streets in the Camden area might suffice, although none are actually squares.