It is divided by signal tracks that GND zone can’t jump over them.
In a case of 2 layer PCB destining one layer to GND makes you have to design it (except GND) practically at one layer. It can seem at first moment very difficult and it is difficult but not so ‘very’.
You should draw all tracks at top (or at bottom - your decision) and when you have to cross them you should use as short as possible track at second layer to not cut GND zone too much.
At your PCB you can probably solve the problem by adding some vias conneting GND zone at top with GND zone at bottom what will give a current a way from one this line end to the second. But it is not a good solution. The much better is to have as much unbroken GND zone at one layer as possible.
I design 2 layers board with continuous GND at one layer. When I can’t avoid crossing tracks I prefer to use 0R for it than to break GND zone. An example of such design:
All vias at this PCB are GND vias. There are 2 or 3 0Rs there.