Bring the money or else the hostage is gone. (the hostage will be killed)
The grammatical sentence would be in the future tense, "the hostage will be killed", which is what you wrote. "is gone" is sort of slang or idiomatic.
I went out and the marker is gone. (the marker disappeared)
If everything were in the past tense, it would be grammatical: "I went out and the marker was gone."
Or, you could pivot into a present tense clause by saying "I went out for a moment, and now the marker is gone."
Or even, shift everything into the historical present: "I go out, and the marker is gone."