(I assume you mean for mlat)
You have to synchronize the clocks used by the receivers to measure the time of arrival.
Using a GPS-synchronized clock to timestamp the arrival time of messages is one way of doing this. This is something that e.g. the Radarcape can provide.
Merely adding a GPS to a system using an existing USB-connected dongle is not enough - the delays introduced by USB are too unpredictable, you don’t know the delay between the signal arriving and seeing the resulting samples, so even if you can use GPS time to measure when different receivers see the samples, that doesn’t tell you the time of arrival of the signal with enough accuracy. (You need an accuracy in single-digit microseconds for mlat to be useful, really)
Another way of synchronizing clocks - which is how this system works - is by using ADS-B aircraft that are transmitting positions as reference beacons.
This doesn’t require GPS.