Gen pop is short for general population. It's used in game QA to refer to the mundane mobs about the world without any goals attached to them. Typically in MMO development the gen pop of an area are given
passes. That is to say a dev will ask QA [or simulate themselves] to identify clusters of mobs, resources, NPCs, etc that are perhaps too dense for a given area, respawn too quickly or not quick enough, react accordingly, etc.
A 'pipsqueak' mechanic is an old dungeons & dragons design trick. The idea being that the
big bad guy of your local area has an infinitely disposable supply of hapless, [somewhat] incompetence minions. Otherwise known as cannon fodder or 'pipsqueaks'.
Think if you will of the stereotypical dragon's horde of clumsy goblins, Darth Vader's crosseyed stormtroopers or the thousand doom bots hurried before Victor Von Doom.
Usually the idea is these pipsqueaks are trivial to kill but numerous. Pipsqueaks can overwhelm by numbers alone if not sort of taken out. This has the psychological effect of making players feel like "heroes" as they cut a swath through the minion ranks. The actual design purpose it serves is to inspire action with a minimum of investment by the player.
For example, in Dungeons & Dragons one might fight through a dozen or so skeletons [which shatter satisfyingly after a few hits] to reach the Necromancer threat. Or in Tera where larger creatures typically have a retinue of smaller minions.
http://www.merriam-webster.com/dictionary/pip-squeak