From Gearing Commander: MSX125/Grom STOCK gearing
Grom:
Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator
Primary Reduction Ratio: 3.350
Gear Ratios:
1st: 2.500 (35/14)
2nd: 1.550 (31/20)
3rd: 1.150 (23/20)
4th: 0.923 (24/26)
Final Reduction Ratio: 3.07 (14/43)
Note that the Grom's 4th gear is an overdrive gear, so that gives it a slight advantage as far as top speed goes since it would run at lower rpms per mph in 4th than the Z125.
Z125 has shorter/lower gearing overall tha the Grom, so should should accelerate better.
I'm sure Ari and Zack on motorcyclist online will have a Z125 soon and do a speed test and review comparison against the Grom.....