Garry's Mod is powerful, but not this powerful. That's where "Photoshop" comes in. But this brilliant Minecraft/Half-Life 2 mashup uses both to tell a charming story.
Before you go asking OMG, where's the texture pack, here's the scoop. Artist Robbaz recorded the opening in Minecraft, went through the portal, then spliced in a recording made in a Half-Life 2 map made to look like Minecraft.
Still, it's nice to see the Creepers get the business end of an M-60.
Minecraft - The Source Portal