There are no complete official stats for .38 Super or .38 ACP in GURPS.
There are incomplete stats for both.
There's a single entry for .38 ACP under the Webley-Fossbery Auto-Revolver.
.38 Super gets mentioned with just about every 1911 description.
In both cases it's just a change to the damage stats, and not to the range stats.
.38 ACP is given 2d+1 pi with the same range stats as .455 Webley.
.38 Super is given 3d-1 pi with the same range stats as .45 ACP.
Is this right?
A 1920 edition of Automatic Pistols on .38 ACP says gives 130gr bullet at 1,175 fps for Winchester ammunition; 128gr at 1,100 fps for Ely ammunition and 130 at 1,000 fps for Kynoch.
Cartridges of the World says 130gr should zipping along at 1,040 fps.
What's that give us?
Using my hybrid calculations of Douglas Cole's terminal ballistics spreadsheet and input from GURPS: Vehicles 2e...
Winchester ammo does 2d+2 pi with a range of 150/1,600.
Ely, Kynoch and Cartridges of the World do 2d+1 pi; same range.
.38 Super shows the same ranges (because the Vehicles calculation is essentially a barrel to caliber ratio modified by a "power" constant).
Cartridges of the World says 130gr at 1,215 fps. That's 3d-1 pi.
Wikipedia references Accurate powder for a 130gr bullet at 1,305 fps. That's 3d pi!
There's a built in ±10% "fudge" factor built into all this. So the weakest .38 ACP load from Kynoch can actually run from 2d pi to 2d+1 pi and the strongest .38 Super from Accurate Powder can vary from 2d+2 pi to 3d pi.
Where this gets a little strange is when we look up the +P rules in High Tech. +P is just a 10% increase in damage, range and ST.
Since .38 Super is often regarded as a +P version of .38 ACP... where does that take us?
Range will change from 150/1,600 to 160/1,800 (because rounding to nearest tens on 1/2D and hundreds on Max).
+10% damage on 2d+1 pi = (((2*3.5)+1)*1.1)/3.5 = 2.514 which translates to 2d+2 pi.
+10% damage on 2d+2 pi = (((2*3.5)+2)*1.1)/3.5 = 2.829 translates to 3d-1 pi.
There you have it, I guess.
My world's "official" stats are going to be:
.38 ACP 2d+1 pi 150/1,600 with ST 9.
.38 Super 3d-1 pi 160/1,800 with ST 10.
Since these rounds physically interchange, what happens if you put them in the wrong gun? That depends.
The old Colt 1900, 1902 and 1903 guns reputably break locking links or the barrel wedge if fed hot ammo for very long at all.
A .38 Super gun might not cycle if its springs are too strong, but it'd be perfectly safe.