Follow these steps:

Create a new repo in bitbucket.

git clone [gitlabRepoUrl]
cd [repoName]

git branch -a
git checkout [each branch-name except HEAD]
git fetch --tags
git remote add bitbucket [bitbucketRepoUrl]
git remote set-url origin [bitbucketRepoUrl]
git push --all bitbucket
git push --tags bitbucket