The very first line of the Infrared sight rules states: ''Weapons with the Rapid Fire (X) or Blast (3"/5") trait cannot be fitted with an infra-sight.''
RAW I'd say the rule clearly says the WEAPON itself can't be fitted with this sight if it has either of the mentioned Traits, so no, you can't mount (aka fitting) an infrasight on a Grenade launcher, even though it would technically work with Krak grenades.
The rule would need to be worded differently to allow what you want, something like 'The infrared sight does not work when used on a weapon/ammo type with the Rapid fire or Blast traits', which is always something your group could House rule if you all agree it would make sense, but Rules as Written, you can't even attach the sight to a weapon if it has those traits.
My group has house ruled that you cannot fit any sight on a grenade launcher, because it doesn't make any sense. Same with throwing knives. And, for balance reasons, we have also disallowed sights on heavy weapons.
I'd personally allow it to be fitted on the weapon and used solely when firing krak/stun grenades (not smoke though, as they should have Blast (5")).
The RAW for weapon accessories allow for completely nonsensical combinations (sighted throwing knives, sighted polearms, silenced hand flamer, sighted long rifle with lowered accuracy...), so they clearly were not carefully thought out and should not be taken as gospel.
Take the autogun for instance. It's a basic weapon so you can fit it with a gunshroud. But duct-tape an under-the-barrel grenade launcher to it (combi weapon autogun/GL) and suddenly the autogun can't be silenced anymore.
The GL has the same kind of issue imo: assume for a moment the GL doesn't come with starting ammunition and you have to purchase it separately: would you allow a GL with nothing but krak grenades to be fitted with an infra-sight? RAW says yes. If you later pruchase frag grenades for that weapon, why should it suddenly remove its sight?