May I suggest throwing out three or four very different ideas you'd love to run for your players and having them choose? I find that approach works well.
Since just about anything is on the table, I'll start throwing out ideas:
We come in "Peace".
Aliens have arrived on earth, bringing high technology and opening peaceful diplomatic relations with earth. Or so they say. They're really prepping for an takeover of earth, gathering information and especially allies among the pariah states and warlords of earth. They intend to arm humans to fight most of their war for them.
The PC's aren't buying it, and they're out to thwart the aliens. A lone scientist knows how to read the emissions signature of the super-science teleportation device, and the characters investigate why the aliens are landing where they are. Their task is to thwart the aliens actions, and hopefully to convince earth these visitors aren't all they claim they are...
Action fits this game well, but is not necessary. I also use the aliens from
gurps space designers notes, but that's for convenience. Use any set you want.
The Sword of Ryland
a quick play I put together. Its designed around its pregen characters, but I feel its worth mentioning.
In a similar vein, The Mook has
a bunch of well documented games he's run at conventions. They're spread across multiple pages, and they use pregens, but they're explicitly made to show off gurps to newbies.
Monster Hunters BIO
In the heart of Africa, and against the background of civil wars, unscrupulous factions experiment on humans in an effort to build new and terrifying weapons, while a group of extra-dimensional aliens find this the ideal beachhead for their invasion of a new world.