While you can use a GPS repeater to see that a GPS device is functional indoors, that will not be sufficient to objectively test the performance (such as the receive sensitivity) of the device. To do that, you will need a GPS simulator and a small anechoic chamber to shield the device from RF interference.
The GPS module might work just fine until designed into the host device, where on-board circuitry EMI might desensitize the receiver or de-tune the GPS antenna. Have you checked that?
Labs such as ours can provide GPS testing and trouble-shooting services during the development stage to verify the design, and we also develop end-of-line production testers which will verify that the device meets performance thresholds before shipping it.