Unfortunately you won’t be able to. 13.6.3 is the last version available for CentOS/RHEL 6 and this is because CentOS 6 and RHEL 6 are now end of life and no longer supported as of end November 2020.
If you wish to use newer version, eg: 13.7.x then you will need to migrate your Gitlab to a newer server release, so either RHEL 7 or 8, or a Debian/Ubuntu based system.
Please also bear in mind, CentOS 7 is only supported for another two years, and CentOS 8 even shorter - this is because CentOS has now ended as a project in the sense of how CentOS was known. Now there is only CentOS Stream which can be considered as unstable/beta/testing.