What are the key differences between the various types of chain tensioners available for gravel bikes, and how do these differences impact the overall performance and maintenance of the drivetrain? Are there any specific considerations that should be taken into account when selecting a chain tensioner for a gravel bike, such as the type of derailleur or cassette being used? How do chain tensioners compare to other methods of maintaining proper chain tension, such as using a tensioning pulley or a derailleur with a built-in tensioning mechanism? Are there any potential drawbacks or limitations to using chain tensioners on gravel bikes, and if so, how can these be mitigated? What are some best practices for installing and adjusting chain tensioners on gravel bikes, and how often should they be checked and maintained to ensure optimal performance?