First thing we notice is that the file given is named CCLocalLevels.dat. This file stores all the data of the local levels I create on my GD installation. My immediate thought is to open it on an online tool such as GDColon's Save Explorer which allows me to see every piece of data for each level in the file, except for the actual level data itself. We see that the flag is in the level somewhere.
I noticed a download button for a .gmd file and downloaded it. Looking through it I realized that it is the level data for just the level (and not all levels like before). I see a really long string which I assume as the level data. Somehow I need to parse this.
Due to prior knowledge, I knew about GD-docs, however a quick google search would find that as well. This site includes all sorts of information related to Geometry Dash's programming including save files. In it I find what the key-value pairs mean and find that I can parse the level data.
It uses a modification of base64 called url safe base64 which replaces certain characters. - -> +, _ -> /. Unfortunately, the result is not readable. I couldn't find how to get past this on gd-docs, but I should've realized that gzip was used like almost every other part in gd (or from the hex).
Instead, I found SPWN-language that directly edited the level data for its purpose, searched for the code that did so, and found this.
I finally got the final string in cyberchef and pasted it into VSCode and replaced every semicolon with a newline to make it readable.
Each line (except for the first), represents a game object inside the level. Each key-value pair is separated by commas.
Immediately, of course, I notice the long base64 message which decodes to the text below consists of multiple text objects. Since there are no text objects in Back on Track (the level they hinted this was), I knew that the flag would be in the texts.
According to gd-docs, the objects with key 31 are text objects. I found each one and put them into base64 which resulted in perhaps try 95484635. I opened up the level (if you don't have GD, you can use GDBrowser). On GDBrowser you can't see the level, but it is just a really short triple spike jump and nothing else in the level.
But we can see anything else about the level on GDBrowser and if we were to open the comments section, we see a 10 month old comment with our flag CSCTF{geometry_dash_d0895c120d671b}.
This is what the text looked like in the level:
Turns out that key 32 scales the object and almost all the text had scale of 0.0f. After scaling it I see
