I guess the biggest difference is that Keptn can act as a control-plane and connect to these tools. Keptn itself has deployment capabilities built-in (using Helm charts) but can be connected to tools such as Jenkins, Gitlab, Spinnaker, ... as well. The Keptn quality gates are comparable to Kayenta in Spinnaker I guess. Although in Keptn it is based on SRE best practices (such as service level indicators and service level objectives).

Another key differentiator is that Keptn takes care of operations as well. Being an event-based system Keptn retrieves (cloud) events and triggers other actions in response to them, such as counter action for Prometheus alerts or problem tickets from incident management system.

full disclosure: I am a Keptn contributor and maintainer.