I love giant apes. So naturally, when it came time to write GURPS Dungeon Fantasy Monsters 1 I was all over writing up my game's giant apes for the book.
One criticism I've heard of the giant apes in that book is that, well, they're just big physical unarmed attackers. If you aren't making rules to ease the problems with unarmed strikes, they can get hurt when attacking. Their defenses aren't terribly high, either. So they are a physical threat but can be diced up by a prepared party (or even a prepared warrior). This isn't really a crushing flaw, though - they aren't really meant to be a "boss" monster unmodified, not against a supernaturally-assisted party of adventurers. For that you need Prefixes and Suffixes, like "Ghostly" or "Juggernaut" or ". . . from Hell" and the like. Still, it does mean they look tougher than they really are much of the time.
One thing they could really use is a way to reach out an touch someone unwilling to come into their (pretty substantial) reach, or who is flat-out dangerous to touch. They've got IQ 6, they're not so dull they can't figure out not to touch the guy in spiked armor or keep trying to punch Mr. Stabby the Swashbuckler. Giving them a tree branch to whack people with can help with the melee part, but what if they want to deal with pesky Scouts and flying Wizards?
I say throw some rocks.
So let's see how heavy of a rock they can hurl at their foes.
Giant Apes have ST 43 and Arm ST +2, for a new ST 45 for throwing and BL 405. What can they hurl at you, usefully?
The sweet spot of throwing is just above BL/2, so you do thrust +1/die and get good range. So this is a rock (or tree trunk) of about 205 lbs (more than 202.5, to be exact).
With a smaller rock, say 102 lbs (over 101.25, or BL/4) damage is lower but range is better.
They don't have the skill now, but that's easily solved - give them Throwing-14 if you're doing to have them use it in combat.
Giant Ape hurling Rocks
Little rock or tree branch (51 lbs.): 5d-5 crushing, Range 90 yards
Small rock or tree branch (102 lbs.): 5d crushing, Range 51 yards
Large rock or tree branch (205 lbs.): 5d+5 crushing, Range 34 yards
To just be mean, allow a thrown tree branch to be treated as a throwing stick and do swing damage. An SM+4-sized throwing stick does sw+2 crushing, weighs 25 lbs, Acc 1, ST 30, and range x4/x8.
Throwing stick: 7d+3 crushing, Acc 1, Range 180/360
How about the big guys?
A King of the Apes has ST 65 and Arm ST +2, for a net 67 ST for throwing and BL 898. In that case:
Little rock or tree branch (113 lbs.): 7d-6 crushing, Range 134 yards
Small rock or tree branch (225 lbs.): 7d+1 crushing, Range 80 yards
Large rock or tree branch (450 lbs.): 7d+8 crushing, Range 53 yards
And the SM+5 throwing stick?
Throwing Stick: 9d+5 crushing, Acc 1, Range 268/536
Sure, with Throwing-14 (and Thrown Weapon (Stick) if you go that route) they won't have a great chance to hit, but it's good enough for harassing fire, and if they hit, you really do need to get out of the way.
Adding the rocks above just makes "I back up faster than he advances shooting him" or "I hover out of range and shoot him" to be a bit riskier. It also makes possible what my extensive video game research tells me is an common ape tactic: standing on top of scaffolding hurling barrels at you as you approach.
* And by the way, if you choose to treat unarmed strikes as a BL/20 weapon instead of ST/10, ape punches and grapples are 20.25 pound weapons, making most SM+0 sized weapons risky to parry with. Stomps should be be treated as a whole-body attack like slam, and thus 43 lbs. For a King of the Apes, it's 44.9 lbs and 65 lbs. Good lucky parrying those with almost any weapon.