Well, I would not count the GK as a defensive force (Only their Fortress and its Garrison). They are more often somewhere in the galaxy than actually on Titan. And even if so it would be the fraction of a Brotherhood.
Certainly they are split up and have troops all over the Imperium ready to respond to daemonic threats but you are missing few important bits.
a) Titan has their main chapter fortress and it is the biggest GK base AFAIK. All space marine chapters have most of their marines going on a crusade or other attacks most of the time but Titan is the place where you are likely to find the most GKs who aren't on a mission.
b) Titan is the most likely base where to find the GK leadership and chapter masters. Being marines these are also the most experienced and among the finest warriors in the chapter.
c) If the attacker happens to be chaos and therefore is likely to have daemons the Grey Knights are the most effective troops against them that the Imperium has. Particularly since "let's just Exterminatus the planet and never come back" is not likely to be an option even if daemons are set loose on Terra.
d) My main point was that forces like GK and Rogue Trader troops and whatever are ALSO defending the Sol system. The system has its own defensive forces just for that purpose (some of which is really over the top like the two titans that are stationed on the sides of the door leading to the Emperor's throne room. Apparently the titans are functional despite looking like statues and have a crew just in case they need to smash their way through the wall that they are partially embedded in and start kicking ass.) but it ALSO has troops and ships that simply happen to be there and will also take part in any defensive battle.