Hello Alex,
Is you project using JS or TS by any chance ? Because on the 8th of April at the end of the day we deployed a new version of the JS/TS security analyzer, that is now able to find much more issues. That would explain why you suddenly see all those new issues appear. Our analyzer was just not able to find them before.
Regarding your quality gate that fails because those new issues are flagged as new code.
Indeed if you are not using the SCM integration then we are not be able to backdate those issues correctly, you can find more information about issues backdating here.
There is no way to manually backdate them, but you could eventually “confirm” them to accept this debt, doing so will update the Quality Gate status and you should be back to green.