My bad, I miss that it was about qlik sense.
So, I am not sure if you can apply conditional colour in the native gauge chart of qlik sense. If you can't, I would try to do a bar chart and play with calculated dimensions.
It is not sth I would liket to do a lot, specially with a big data set, but if you really need it and don't intend to use an extension, maybe you could do it.
The first dimension is a plain text. The second one is a value list. The first one represents your expression and the second the gray area.
After that you may be able to work with the conditional for the value list equals to your expression area
Than you can hide axes and make it look better.
It is not an amazing solution but may work for what you want.
so, following the images I sent before you would have sth like:
=1 (or any text you would like)
=Valuelist(1, 2) (or any two names you would like
,[Your Expression that returns a number between 0 and 1]
, 1 - [Your Expression that returns a number between 0 and 1]
if( (valuelist(1,2)=1) and ([Your Expression that returns a number between 0 and 1]<0.8)