Using a github code for my journal paper
It would be plagiarism only if you don't cite the source. There's no plagiarism issue if you properly explain that you use this source code as a basis for your implementation and make it clear that it's not yours (preferably by citing a paper from the original authors if any, if not just give the github link).
Whether this will be considered an original contribution by the reviewers is a different matter. It might be ok if your motivation and data are significantly different from what the original authors did.