is it possible to connect several measuring amplifiers to the PC with USB?

Operation of several GSV-4USB or GSV-8DS on the PC and use of the synchronization cable

This is not a problem.
A virtual serial port (VCOM port, virtual COM port) is assigned to each GSV-4USB or each GSV-8DS.

With higher data rates, synchronization of the two or more GSV-4USB or GSV-8DS is recommended.
To do this, one device is configured as "master" and the remaining devices as "slaves".
The digital output of the master is connected to the digital inputs of the slaves.