I am in exactly the same position, my BRITPART rear silencer just fell apart after two and a half years gentle use. The cause was that the pipe rather than the silencer just disintegrated, the silencer is fine, the pipe is just a cylinder of ferrous oxide.
Like you I have been supplied with a GT Exhausts replacement, I have never heard of them, but they would have to try very hard to be worse than the one that it replaces.
Of note is that the length of the GT Exhausts pipe is slightly shorter than the BRITPART pipe in front of the front hanger, this may cause some issues if you are mating it to a BRITPART centre pipe, in my case it fitted better, but that may be because it is a 300tdi to LT77 conversion with a modified front pipe.
I know you get what you pay for, but I was genuinely surprised how 5hite the BRITPART silencer was.