If I understand right, what you need is the interval (lower, upper)
over where the prob from your truncated gamma is 95%, and the prob for interval (4, lower)
is 2.5%, and for interval (upper, 6)
is 2.5%. If so, by straightforward algebra:
R > F = function(x){ pgamma(x, shape = 50, scale = 0.1) }
R > F(4)
[1] 0.07034
R > F(6)
[1] 0.9156
R > gap = 0.025*(F(6)-F(4))
R > gap
[1] 0.02113
R > (lower = qgamma(F(4) + gap, shape = 50, scale = 0.1))
[1] 4.087
R > (upper = qgamma(F(6) - gap, shape = 50, scale = 0.1))
[1] 5.9