When gausian blurring image that is left half red and right half green there appears somewhat of black border instead of liquid transition from red do green.
In photoshop I can use LAB Colorspace and it is ok, but I haven't found anything for GIMP to achieve such effect.
Is there any method to achieve effect like in LAB colorspace in PS?
If you have the G'MIC plugin installed, then yes.
With that plugin installed, you can click Filters > G'MIC > Degradations > Blur (gaussian), and set the channels to Linear RGB (all)
Good explanation of the problem here.
If you can use the current development version of Gimp (2.9.x) you can work in "Linear light" (ie, the equivalent of "squared" in the video) and the blur works as you expect:
