GURPSよろず

Long Weapons in Close Combat
You can try to strike with a non-close combat weapon (anything without a “C” in its Reach statistic) in close combat, but this is awkward. You must hold it out to one side, make “wrap” shots at your opponent’s back, swing at sharp angles, or thrust almost straight up or down. Still, sometimes it’s better than the alternative!
A reach 1+ weapon in close combat gives a skill penalty equal to -4 times its longest reach in yards, regardless of the reach you were using it at. This is -4 for 1-yard weapons (most one-handed swords, flails, and impact weapons), -8 for 2-yard weapons (spears, staffs, short polearms, most two-handed flails and impact weapons, and longer swords – greatsword, katana, rapier, etc.), and -12 for 3-yard weapons (long spears and polearms). This lowers skill for all purposes – including making and resisting disarms and feints, and all armed striking techniques – except those specifically excepted below.
You can parry in close combat, but this is also at a penalty. Calculate Parry from your reduced skill. Thus, a weapon parries at -2 in close combat if reach 1, -4 if reach 2, or -6 if reach 3. This doesn’t affect parries made as your foe enters close combat. Likewise, if you’re able to retreat out of close combat, you get your full Parry plus the usual retreat bonus.
Thrusting damage is unchanged in close combat, but it’s hard to get in a good swing. Reduce swung damage by -1 per yard of your weapon’s maximum reach. For instance, a broadsword has -1 swinging damage; a two-handed sword, -2; and a halberd, -3. In addition, a reach 2 or 3 Polearm, Spear, or Two-Handed Axe/Mace weapon can only do quarterstaff damage with the haft (swing+2 crushing, less the reach penalty) unless you take a Ready maneuver to choke it around the head. It takes another Ready to prepare it for normal use after leaving close combat.
These effects apply to all weapon use, offensive or defensive, while in close combat. It doesn’t matter whether your current target or attacker is in close combat with you! Just having someone in the way is enough to make things awkward.

Exception: Pummeling (p. 111), grappling techniques (Armed Grapple, Bind Weapon, Choke Hold, etc.), and similar close combat-specific moves don’t suffer skill or damage penalties in close combat. See Defensive Grip (pp. 109-111) and Reversed Grip (pp. 111-112) for other ways to use long weapons in close without these penalties.