/etc/gosa/sieve-spam.txt is in gosa-plugin-mail 2.7.4+reloaded3-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | # Sort mails with higher spam level
if header :contains "X-Spam-Level" "$spamlevel" {
fileinto "$spambox";
}
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