I'm not a FWL fan, but I actually rather like the Light Gauss, especially as a refit for older AC/10 carrying units, giving them a role as cheap direct fire support units. Take a few CN9-As with some surplus Imperator IIs replacing the old Luxor-Ds and you've got a nice group of snipers.
I rather like MRMs as well but that's because I'm a long time C3 player. If not playing with C3, though I'd prefer to have them on units with high mobility. (imagined a salvaged Blitzkrieg with an MRM40 and a Supercharger to put the fear of Itano into someone...)
This guy? He gets it.
Remember, the LGR is a Marik weapon. VERY few Marik units are designed to just be fantastic right out of the box- they're meant to work as part of a group. So what looks often like an underwhelming weapon, or platform for that weapon, becomes a part of a larger picture- one that can become absolutely terrifying if built and used right.
So yeah, the LGR is a big, heavy, clumsy weapon that only does eight points of damage. It has great range, yes, but still, eight points. Four of them though from the Centurions mentioned above? That's a little worrisome- that's four hits coming in at ranges where you may not even be able to respond, or at least where their medium range bracket starts where your long range is still going on. That adds up fast, even on an assault Mech. Now back that with signature Marik LRM support, some close-range muscle in case things get in under the umbrella... and a company with an LGR lance becomes a very tough nut to crack.