Looks interesting. RigExpert makes excellent antenna analyzers. This GPSDO is about 2x the price of a BG7TBL but it has some extra features including the 4 outputs which would be useful if you have more than 1 device to drive. On the other hand you could add an external BG7TBL distribution amp to the BG7TBL GPSDO for about $100 that would support 8 devices. Might come down to the reference clock specs.
From a BG7TBL eBay ad:
After GNSS is locked, frequency accuracy can reach 5E-9 (0.05Hz@10MHz) within 5 minutes. Finally, accuracy will reach 2E-11 (0.0002Hz@10MHz)