Non-code issues
Quite a lot of issues that arise during the project's development process are not related to writing new code or adding something to the project. Especially at the beginning, these often involve learning LibGDX, creating project plans, or holding meetings with the team or supervisor. These issues also need to be marked and time spent on them. Here are some examples of how they could be marked.
For meetings, for example, you can create an issue for each sprint/milestone where all team members can write down the time spent. You can also write comments there about what was discussed during the meeting. You can also add meetings held with the supervisor.
If there is any documentation, material, or learning from a YouTube video, you can also create a separate issue for it, or you can add time spent under the issue for which you watched/read that tutorial. For example, if you're learning how to calculate the player position on a map and watched a video, you can write a comment in the player position issue, where you add the YouTube video link you watched, and write down the time spent. If it's more general learning about LibGDX, where you read documentation, you can create an issue for getting familiar with LibGDX and write/link what and what you read from the documentation and how long it took.
Creating an issue for organizing GitLab and planning for the next sprint is also possible. Here you don't necessarily have to comment exactly what was done or organized, but you can write generally. Also, if after creating this issue you can see that the next milestone has emerged, which nicely lists the issues you plan to do, it's also visible that sprint planning has been done.
In summary, write down the time spent on each thing. It's always better to create more issues and record the time spent on each thing. Whether it's writing code or learning or even creating issues themselves.