Yes definitely you can apply for ssc cgl 2019 even if you are completing your graduation in 2020. The only eligibility criteria required for the exam is bachelor's degree or equivalent from a recognized university and board. The exam registration has a very long process. So just do submit all the required certificates properly and safely. The exam procedure is too long. Till now, CGL 2016 is about to take place and CGL 2017 is not yet out only. So maybe once you come in 2020 after that only CGL 2019 comes out so you have a high chance to apply.

Best wishes. Thank you.