So my Question is:
Who are other enemys/dangers for the gremlins (so except the Spiral knights...)
I mean they had already all these weapons etc before we crashed...
Did they attacked Strangers?
What did these strangers do except seeling us things... i mean they had a life before we crashed too
You should probably send these sorts of questions to Spiral Knights Support if you want to have any hope of an official response.
The Gremlins probably had those weapons for the same reason the US kept its massive military around after World War II; paranoia. As for what they were paranoid of, consider the jabbering the speaker in the Sanctuary does about the "pact" and how it's imprisoned by the Clockworks. (Yes, the speaker is the Core.)
Strangers don't really have a role in my opinion; they're more lore dispensers than anything else.