In CT, level-0 skill means no penalty, while no skill imposes the penalties. So the game expects that Level 0 gun combat has had to fight using a gun, and at least viscerally (pun?) knows what it means to be in a firefight.
I think part of my point was building from earlier in the thread - to me, "Gun Combat" as a skill covers a broad swath of information, from the weapon itself, to basic tactics using that weapon (not general "tactics" as a skill), to why you shoot rounds where on a particular sophont....
Can't agree with this thinking.
Just look at the rules. Gun Combat skill provides no DMs for achieving surprise, for morale checks, for damage ... it affects only one thing: to hit.
Let's recall that in CT there was originally no "gun combat" skill. You had Shotgun-1 or Auto Pistol-1, etc. This was silly on its face; if you are skilled with a handgun then you ought to be skilled with any handgun. So Gun Combat generalizes this skill.
Calling it "Gun Combat" confuses things because of that "combat" word ... but the default Traveller assumption is that guns are for fighting. It's still just marksmanship, because marksmanship is the only game effect.