hi @Miguel1900
in BaseEditor.ini:569:
[/Script/UnrealEd.AssetViewerSettings]
+Profiles(ProfileName="Default",DirectionalLightIntensity=2.620000,DirectionalLightColor=(R=0.990000,G=0.839850,B=0.732600,A=1.000000),SkyLightIntensity=0.880000
So its reasonable to say its been set to
SkyLightIntensity=0.880000 initially
in BaseLightmass.ini
bUseFilteredCubemapForSkylight=true
[DevOptions.ImportanceTracing]
AdaptiveSkyVarianceThreshold=.5
bUseRadiositySolverForSkylightMultibounce=True
[DevOptions.IrradianceCache]
; Sky occlusion has less noise than normal GI, dont blur away details
SkyOcclusionSmoothnessReduction=.5
[DevOptions.StaticLightingMediumQuality]
AdaptiveSkyVarianceThresholdScale=1
[DevOptions.StaticLightingHighQuality]
AdaptiveSkyVarianceThresholdScale=.5
[DevOptions.StaticLightingProductionQuality]
AdaptiveSkyVarianceThresholdScale=.5
So you can see there is a lot to play with, but LGPU probably is a combination of the values. When looking at the code this seems to be set to use only Production Quality.
a float of 0.50 is hardcoded throughout the CUDA code.
There is no way to adjust the as its fixed value into LGPU
CPU Lightmass says
Note that the physical lighting units are measured in cd/m2, like the Sky Light or Emissive Materials. Sun and Sky sources have that range in the thousands of units, which it does hand-in-hand with physical cameras where the Exposure Value (EV) are in the range of EV100:14 (see the "Sunny 16" rule). However, with the HDRI Backdrop asset, it is not required to use physically correct values, but you may need to set the EV to something significantly lower than the EV100:14. It's also worth noting that some HDR images range from 0 to 5.0 or greater than 5 cd/m2 while others are ranging from 0 to 100K units. This means that when you swap HDR images, there may be a noticeable difference in brightness changing.
Epic GPU Lightmass (EGPU) 
5.3 documentation says
Stationary Sky Light is Not Supported Support is planned in a future release of the engine.