Angry Tools
It appears that you can use this site to set up any combination of flex properties that you need (I don't see gap, but that's not specific to flex anyway). Besides toggling the normal flex properties, you can set the order of flex items directly as well as add a seemingly infinite amount of new flex items. Also, there are 53 preset scenarios you can use. After everything's done to how you need, you even have the option to copy the code. Plus, there's a short flex explanation below and an inactive discussion board.
W3 Schools
W3 Schools is always a good resource for working through any web concept step-by-step. You need to visit a few pages, but there are plenty of different visuals, demos, and explanations. You can even take the little quizzes at the bottom of each page if you want to test yourself. As for the demos, if you go into them knowing some code, you can change everything around to anything you want. I wouldn't necessarily use it as a full sandbox, but you can alter the concept you're researching.
CSS Portal
This kind of does the same thing as the first resource, but I wanted to include it due to a few differences that make it both better and worse. First, you can apply a bit of non-flex styling to see how the elements would get affected due to different sizes. There are global margin and width values, and selecting an element gives you specific width and height values. Another value Angry Tools didn't have is the ability to experiment with inline-flex, which doesn't make too much of a difference, but it's still good to have if you're going to copy the generated code and you want to use inline. Something I don't like is there isn't much space to work with the properties on my small laptop screen because the visual box scrolls with the page. I get that they need a way to make sure you see the values you're changing update in real time, but they could've done something more like Angry Tools.
Summary
There are a lot of similar tools for flex out there, some better than others, but they also tend to serve different purposes. If there's something you specifically need that one resource doesn't have, there is probably another one that does, or at least you could use a sandbox.