What I understand is that you would like to show traffic lights based on the value as follows :
Less than 50 -> Red
Equal to 50 -> Grey
Greater than 51 -> Green
If this is your requirement, then here is how to achieve it.
1 - I have put min vlaue 0, max value 100.
2 - Created 3 segments
3 - All options of Guage properties are un-checked except 'Relative segment bounds'
4 - Put 0,0.5, 0.51 as lower bounds respectively.
5 - Click OK
This should show red light for values less than 50, grey for 50 and green >50.
I hope this helps!