Hi,
Welcome to the community, @chandraprasath!
Normally, we try to keep it to one topic per thread. Otherwise it can get messy, fast. I’ll make a pass at your multiple questions here, but if you have follow-ups, I reserve the right to ask you to create a new thread. 
Quality Gates can be administered (only) through the SonarCloud UI, by an administrator. You can have as many Quality Gates as you like, but we consider it a best practice to try to unify all your projects under the same Quality Gate. That said, internally we have multiple Quality Gates because some teams wanted to impose stricter Coverage standards on themselves than the company norm.
You can’t. SonarCloud doesn’t support plugins. What you can do is obtain an OWASP report before analysis, get the results into the Generic Issue Report format and feed that into analysis.
We’re working on adding this type of functionality, but in the meantime, you would need to use the web APIs to pull the data and craft your reports.
The best way to master the API is to perform the desired action via the UI and eavesdrop to see which calls the UI made to accomplish the action.
You may also find this guide helpful.
HTH,
Ann