There are a few places I’ve found with lots of prebuilt monsters for GURPS, though they aren’t really consolidated in one place. I usually look through one if the following if I want something prebuilt:

• the creatures of the night books (mostly solitary monsters you could design an adventure around, with suggestions for each one)
• the fantasy bestiary (lots of generic and mythological monsters from all over the world, with stats and descriptions for each one)
• dungeon fantasy monsters 1-3 (lots of monsters, kinda built with dungeon fantasy settings and power levels in mind)
• the gurps repository also has tons of free and generic statblocks