Sears is located at the Mall at Rockingham Park and the gulls gather in the lot between the Sears Auto building and the main Sears entrance to the mall. Map: https://goo.gl/maps/DjNvJQ4Ygy12
The gulls tend to gather here in the morning and around noon into the afternoon (I think someone feeds them here now that Rockingham Park is closed off).
If unsuccessful at this spot - other places to look nearby are both Market Basket parking lots and the Target/McDonalds area along Rt 28 - often perched on top of Target. Also worth checking would be Canobie Lake or Hedgehog Pond.
There are also increasing numbers of Ring-necks on World End - 100+ along with 3 Common Goldeneye, 2 Bufflehead, 1 Lesser Scaup, 1 Wood Duck and 2 Mute Swans.