This just occured to me- I don't know if it's true or not, but I think I remember hearing on a History Channel show that gastraphetes were used in the siege of syracuse. So my (extremely tentative and unsure) answers are:

1) Yes- but primarily for siege or naval combat since they were bulky and worked best in cramped quarters

2) Probably- Caesar isn't the biggest fibber out there, and there's no reason for him to lie about the German projectile choice anyway, so I'd say it's a safe bet to assume the Germans used them.

But I'm no expert, maybe a real historian could come in and offer their advice