GURPS melee treatise vol. 3: equipment and advantage bonuses

[elglin]

All those fancy attack options and complexities make sense only from a certain skill level, and there isn't such a thing as "too much skill" when talking of a realistic campaign. Unfortunately, the vast majority of those things are rather exotic/supernatural (e.g. Altered Time Sense) or cinematic (even the seemingly innocent Enhanced Parry/Block/Dodge), so, in a low-magic realistic setting, your options are few in between.
There's the obvious way to raise your attributes (if allowed) and skills (if allowed without limit), but here I look at the options besides those.
Defense
The Shield skill. No, really, it is probably the best defensive investment of a skill point in the entire combat system. You gain an extra defense type (Block), you gain the shield's defensive bonus (anywhere from +1 and +3 to defense) and a Shield Slam as a Move and Attack option which allows you to sort of attack the opponent at your full skill. Depending on the intangibles, you may leave this at 1 point just for the shield bonus or actually treat your shield as a "secondary weapon", thus improving not only your Parry, but your Block as well.
Speaking of shields, a buckler or a small heater shield is DB 1, a Viking round shield, a large heater, a kite and the smaller legionary shields are DB 2 while only the full-blown legionary shield (or the pavise) gets DB 3. With the latter, the Shield-Wall training perk is obligatory as it erases the associated -2 malus to attacks.

Combat Reflexes. Just about the only thing allowed in a realistic campaign to boost defenses, giving +1 to all of them and some assorted benefits. This represents an assortment of well-trained muscle memory responses to a variety of situations which comes with either extensive training, extensive experience, or, more likely, both.

And that's all, folks!

Attack
Well, it's not DnD, so kiss goodbye to your +5 Vorpal Sword.

Weapon Bond perk. "There are many rifles, but this one is mine". For a weapon of above-average quality (balanced or fine or both, see below), this gives a +1 bonus to skill for this weapon only and to that character only. Everything and everyone else, including another weapon of the same type or a twin sibling, doesn't get that.
On the subject of weapon, it can be balanced (+1 to skill, more precisely, "balanced for this particular fighting style"), fine (+1 to damage) or very fine (+2 to damage) for more or less exorbitant cost. So, taking the example of Excalibur, it could have been a very fine balanced weapon (making it more or less unique across Britain) for which Arthur certainly had Weapon Bond, for effective +2 to skill and +2 to damage (which is a measly +2 sword in DnD). Given that GURPS characters don't get hit dice and a constantly growing HP pool, the damage bonus is actually way less important than in DnD.

Weapon Adaptation perk. This isn't really a bonus to the attack, rather an option to save investing points in another weapon skill. With that, for a measly cost of one point, you can e.g. use your Broadsword skill to use a greatsword, saving you quite a few points while retaining efficiency. Real-life martial arts pundits would rightly squint at this, but a) "it's just a game, why you heff to be mad" b) it definitely makes some sense, at least as far the medieval treatises (e.g. Talhoffer) are concerned.

Armor and DR
In GURPS, unlike DnD, armor behaves as damage reduction, which it really should. Of course, if one uses the expanded rules, you can make your armor as thick as you like and as high-DR as you like - for the added price of being over-encumbered by carrying too much weight. In practice, there are two options.
Armor can be fine, which is a weight reduction which, grossly simplifying, means "DR-1" weight for "DR" armor - or, conversely, +1 DR as compared to normal. This is not a stretch of imagination - a pretty long essay I've recently read suggests that late Roman metallurgy (in terms of metal purity) and craftsmanship (in terms of work hardening) was unsuccessfully chased by medieval and Renaissance armorers, finally catching up only in 16th or 17th century.
There's one other option, and that's face hardening. It was much used by Renaissance armorers in the 15th century - abandoned later for presumed increased brittleness especially when confronting bullets. There is material evidence of face hardening on Roman medical instruments. With the metallurgy of the day, it wasn't an option for large pieces like a body plate or even the large elements of a segmentata; however, scale armor, attested in the Eastern Empire and definitely, if infrequently, used by certain auxiliaries in the Western one, would be amenable - if very expensive - to such a treatment.

Bottom line, returning to the Once and Future King, he may have (and as per Bernard Cornwell, had) possessed, in addition to Excalibur, a suit of scale not unlike the contemporary Byzantine ones, but fine and hardened.
Which, all in all, means, that in game terms, over and above an equally-skilled adversary (who were few and far between anyway), he had +2 to attack, +1 to defense, +2 to damage and +2 to DR. Each of these is a pretty big deal, as far as the GURPS system is concerned, and all of them combined would thus give him a quite tangible advantage.