Gitlab have concepts of branching and merging. Is there any ISO standard or any other standard which gives standard guidelines to use branching and merging in new projects?
No, there are a number of common branching strategies that are used by different projects and companies. There’s a summary of a few models here and you can find many more around the web. What works best for you will depend a lot on your circumstances, so it’s hard to give concrete advice about this.