It is not the entire gospel, but it is a part that we are to preach to animals because that is the part that they can understand, and is linked to how the second coming of Christ will affect them.
Preach the Gospel always, use words if necessary. -- St. Francis.
At the very very least, kindness is an indication of spiritual health and cruelty one of spiritual dis-ease.
It is ALWAYS necessary.
1 Corinthians 1:21
For after that in the wisdom of God the world by wisdom knew not God,
it pleased God by the foolishness of preaching to save them that believe.