You shouldn’t have to upgrade CBL or Sync Gateway. Minimal support version of Android API on CBL is API22 and SGW is completely independent of the CBL OS version.
If you are having issues, let us know.
That said, 2.1 is a very old version and I’d strongly recommend that you upgrade to the latest 2.8.1 version of CBL - the minor releases are backwards compatible and the upgrade should be seamless (the only change is the init() call as specified in docs).
Note that upgrade of CBL to 2.8.x does not require corresponding upgrade of SGW to 2.8.x if that’s something that cannot be done be done right away. So you can stagger the upgrade.
Related -