Values are what someone will pay, which, as has been stated in the thread depend on location and freight costs etc. I could not suggest any value that you might realise in the USA.
I can give a little background information on the G&J clocks (and their contemporaries) which may help. In the UK there were four 'main brands' of 1 second pendulum gravity arm style master clocks which had a 30 second impulse to drive slave clocks. Although there are differences, they are broadly compatible as regards slave dials.
The most common were Gents and Synchronome, both of which were made in some quantity roughly for the period 1910 to 1980s. They change hands frequently on the major auction sites and so a 'value' can be obtained there. Generally speaking Synchronome clocks fetch slightly more as they are considered a little nicer to look at and are slightly less noisy. Both are excellent and reliable timekeepers with invar pendulum rods.
The less common are G&J and ECS (English Clock Systems, a division of Smiths Industries who made large quantities of clocks and watches).
ECS were made from roughly 1945 to circa 1960s - and though quite lot less common than Gents or Synchronome are only a little more 'desirable' as collectors items (rarity value). They can be a little less easy to set up and get running.
Finally G&J. The first clocks were introduced in the late 1920s and were usually in a rather elegant case with a slightly pedimented top similar to early (pre 1920) Synchronome clocks. The mechanism was a little different in that it had been designed such that it was quieter running than the other clocks due to the design of the solenoid having a longer but gentler movement. Early clocks had a similar solenoid arrangement for the pilot dial. These pedimented case clocks were soon superseeded by a flat top case design similar to the contemporary Synchronome and pilot dial movements changed to a more conventional design (like Synchronome). Most G&J clocks are found in this case flst top style. After WW2, the 'round corner' case identical to the contemporary Synchronome was used, but relatively few clocks were supplied in this case style probably through the 1950s into the early 1960s. This is the style of the original poster's clock. At the time - the G&J operation was under the same ownership as the Synchronome company.
Overall G& J clocks are relatively scarce - and the slightly quieter running makes them a little more attractive - and they are nice reliable clocks and keep time well. They will drive 30 second single polarity (Synchronome, Gents, or ECS) slaves as well as G&J slaves.
Unfortunately there are no serial numbers or records, so accurate history and dating is not possible. Generally in the UK G&J clocks sell at a significant premium over a similar clock made by Synchronome.