They do absolutely nothing and give zero threat to the enemy. They are on 25mm in a 4-man unit. Therefore, they attract little attention and can be easily hidden on an objective. For 30pts a squad, it's the cheapest thing we have and it makes a lot more sense to leave those on the backfield objective than to waste a
GKSS on it. (Although the
GKSS are a good anti-deep strike deterrent).
As for the list, have you considered taking two Brotherhoods? Rapiers is really good for Infantry, which you have a lot of. T1, double move the GKI, T2 use the Rapiers power to power up and charge.
Prescient Brethren is good utility, as they have a fantastic
WL trait (we are very
CP hungry) and a solid stratagem for those GKI who will be far away from support auras.
Servant of the Throne Gift, instead of Foretelling of Locus, makes your GMDK much more tanky. T1 he can teleport with the Sigil, hopefully out of
LoS; then T2 he can activate Servant and take a load of shots to the face. If you really want the Locus, it goes better on another character, although I see that you only have the two.... Maybe drop the small GKI squad to fit in a Chappie or a Bro-Cap. That gives you another cast, and with all the
PAGK Sanctuary would really help you out.
Finally, since you're giving the Libby only one Dominus power, it might be a waste to give him the Gem. That's taking an upgrade slot AND some points for a once-per-game +2 to cast on Vortex (it doesn't work on Purifying Flame). Since it doesn't explode any more on a high roll, that might not even be necessary.