Una,
Hmm, it could be Gan was the first human subject they tried it on, and they decided it wasn't a cost effective way of going about things - perhaps they realized the limiters would have a short working life (it breaks down pretty quickly, doesn't it?) and they'd be better just executing or imprisoning
violent
prisoners.
In Trevor Hoyle's novelisation of the first four episodes, _Blake's 7_, Gan explained to Jenna in Chapter 11 that the limiter implant took place before his trial because 'They needed humans for their research, so I was just handed over'. There is an implication there that he was not the only person experimented on.
Regarding your own view that it would have been better just executing Gan, I have a belief that he got a reduced sentence due to the implant, on the grounds that he was at least doing something useful for the cause of science. In 'Breakdown', when Renor expressed his disgust at Gan's limiter implant, Kayn asked if he would have preferred that the latter had been executed.
Murray