It's hyperbole to say you can use it for the entire lifecycle. That might work fine for one-product monorepo shops, but Gitlab still doesn't scale well right now. Aside from basic issues like the runners not working well, the binary artifact repositories are way behind what something like Artifactory or Nexus offers. It's extremely annoying not having group and server scoped registry tokens except for the container registries. It makes it more difficult than it should be to publish modular libraries to be used elsewhere in your organization by other products. It's effectively unusable if you're working behind an air gap and trying to mirror public registries. The only way I can think to do it is creating a dummy projects with every kind of package registry enabled and push all of them into that one repo, but that won't work for things like Maven that have a notion of namespacing.
It can't really replace something like Jira, either (and in Platform One's case, it doesn't). You can only create an issue in a repo, but there is plenty of work organizations do and want to track and organize that can't be directly tied to a code change, let alone a code change in only one repo. So where do you raise an issue to track work not related to writing code or related to writing code but across multiple repos? I've seen people create dummy repos that serve no purpose except being a central place to put issues, but that is just working around the limitations. It's fine as a bug tracker, but not a general purpose work tracker and project management solution.
Also, in Jenkins' defense, it's often nice to have a general purpose automation server. I never liked Jenkins, but I miss being able to create jobs that check the health of your deployments, report filesystem usage to user of your developer workstations, run very large-scale end to end integration tests independently from builds, update wikis and documentation automatically. There is plenty of automation that can happen outside of code CI that may not be related to code changes at all but is still useful.
Understanding of course Gitlab does have a notion of scheduled jobs that just run on a timer rather than being triggers by a changeset push, but that still isn't enough, and embedding shell scripts in yaml strings is a very poor substitute for Jenkins' Groovy DSL when there is any kind of complicated logic required by your jobs.
Gitlab does have some features like rules and triggers that can do medium-ish complexity flows. Still not where Jenkins is, but there's some framework pieces there.
It's hyperbole to say you can use it for the entire lifecycle. That might work fine for one-product monorepo shops, but Gitlab still doesn't scale well right now. Aside from basic issues like the runners not working well, the binary artifact repositories are way behind what something like Artifactory or Nexus offers. It's extremely annoying not having group and server scoped registry tokens except for the container registries. It makes it more difficult than it should be to publish modular libraries to be used elsewhere in your organization by other products. It's effectively unusable if you're working behind an air gap and trying to mirror public registries. The only way I can think to do it is creating a dummy projects with every kind of package registry enabled and push all of them into that one repo, but that won't work for things like Maven that have a notion of namespacing.
It can't really replace something like Jira, either (and in Platform One's case, it doesn't). You can only create an issue in a repo, but there is plenty of work organizations do and want to track and organize that can't be directly tied to a code change, let alone a code change in only one repo. So where do you raise an issue to track work not related to writing code or related to writing code but across multiple repos? I've seen people create dummy repos that serve no purpose except being a central place to put issues, but that is just working around the limitations. It's fine as a bug tracker, but not a general purpose work tracker and project management solution.