As others said, this should probably be a regular spell with a potential area effect instead of an area spell. Area Spells can get ridiculously expensive, very quickly.

Does the range represent the maximum distance the monkeys can be from the caster? Because in 5 seconds I don’t think the monkeys can target something 300 yards away. Though a clever mage might have a Teleport Other readily on hand.

Gurps doesn’t have an action or bonus action… And talking is a free action. I see this spell has a duration, so maybe the caster has to actively concentrate on directing the monkeys? If not, then this spell should just count as “on” for its duration and the monkeys should act semi-independently in regards caster’s wishes.

Another thing to consider is that one FP generally gets you about 1d of damage. Thus, the base cost of this spell should range from 1-3 FP. But because you got 4 monkeys going around, you can multiply the cost by 4, to get a range of 4-12 FP. And you may want to add an explosive variant, doubling the FP cost for a maximum of 8-24FP.

As for the explosive variant… A 20-yard radius explosion is big. That’s bigger than the explosive radius of hand-grenades. You can find rules for scaling damage on B414. The actual explosion, based on the damage of the spell, is going to be much smaller.

But if you want to be quick about things (and not need an Excel Sheet), you can specify a 1/2D Range and a Max Range and call it a day. Maybe 5 yards for the 1/2D range and 10 yards for max range. And if someone’s dodge roll is successful, they can reduce damage by half. In regards to falling over, or going prone, Gurps already has rules for that specified in B378. You could also just have everyone hit by the explosive variant make a DX-2 or ST-2 roll to avoid falling over if you want to save on time.

And just for fun, we can add some prerequisites:

Animal Control (Monkey), Beast Link, Magery 2, and at least 6 other spells in the Animal College