"But Henrique," - I hear your woes - "why would you want to use an Open-Source Game Engine? And an Engine with an Editor?".
The advantages are simple, young cricket:
It's the best of two worlds: you can delve deep in the engine code if you want, but you won't have many obstacles starting out with the Engine to create your dream game.
There are many Open-Source Game Engines. There are many Game Engines with Editors. But here are the best engines that have both open-source code and an easy-to-use Editor. We choose the Engines based on how stable they are (how up-to-date the Game Engine is?), how popular they are (how big the game development community is?), and we selected only the Game Engines in which you will work on a visual editor most of the time.
Without further ado, here are the engines:
Godot is easily the top-1 pick.
It's fully open-source, publish to many platforms, has one of the most stable and fully-featured editors out there (often compared as an Unity Game Engine alternative), and has tons of learning content and courses available. The source-code is a huge C++ project and it might look scary at first, but it's a great learning reference for sane C++ standards in a single codebase. And if you just want to create a game fast, Godot is packed with it's own easy-to-learn scripting language fully ingrained with editor functionalities, so you can start building your game fast.
If Godot is an alternative to Unity, GDevelop is an alternative to Construct 3. It's one of the game engines with the most friendly editors, and is packed with a fully-featured visual scripting editor in case you want to start building your game even faster, without having to program a single line of code. And sure enough, visual scripting is much more limited than programming - but GDevelop makes it easy to extend its own visual scripting with plugins you can download for free. And obviously GDevelop is Open-Source, so if you want to go knees deep into Javascript and extend the engine even further, you can!
Defold is a professional-grade Game Engine rising in popularity as a possible Unity alternative. Like Godot, it has a mostly C++ codebase, publishes to many platforms and uses a scripting language - but instead of creating it's own language, Defold integrates Lua, a well-known scripting language used in many other games, and thus having a lot of existing content to learn and libraries to extend the language if needed.
Twine is a Game Engine for the very specific genre of interactive stories, so it doesn't need to get complex - but the editor makes creating interactive stories very easy visually, allowing users to see the "connection" between differet story sections and what is the flow of the player/reader. It's also very easy to extend the editor using Javascript to add elements to your interactive fiction, with plugins to even include RPG-like mechanics!
Another Unity-like contender, Stride is a C# to-the-bones Game Engine, so if you like the .NET ecosystem, Stride has the amazing advantage of being built on top of C# and to also use C# as it's scripting language. That means that learning how to make a game on Stride also allows you to delve more easily on the Game Engine source-code, making the process of fixing and extending the Game Engine much more straightforward.
GBStudio is a Game Engine that allows one to build simple games without having to code a single line of code - which is great, because we're talking about a Game Engine that can export your game to a real Gameboy, and this is NOT an easy task. That also means that GBStudio is a good fit for a handfew of genres that could work on a real Gameboy - arcade games, simple top-view action games, etc.
PlayCanvas is a "web-first Game Engine" - that means that it can publish games to Web, but also that it's editor is also web based! Not only that, it allows more multiple users to work on the same project at the same time, a "Google Docs" sort of approach to game development. Unfortunately the Editor itself is not Open-Source, but the Game Engine is (so it still meets the requirements of this list), and it's one of the few 100% Javascript codebases, not even a sprinkle of other languages.
Another "web-first" Game Engine, microStudio is PlayCanvas much friendlier but less feature-packed cousin, but unlike PlayCanvas, microstudio is fully opensourced (Game Engine, Editor and all). microStudio also have an amazing in-editor interactive tutorial, and you can pick between 4 languages to script your game with (microStudio own scripting language microScript, JavaScript, Python and even Lua!).
A Game Engine with an undeservedly low popularity, Armory3D is a Game Engine that you install as a "plugin" for Blender3D, extending the popular 3D modelling software with an amazing set of features to work with 3D games using the Haxe programming language, and even allowing programming with visual nodes (similar to Unreal Engine blueprints).
One of the most interesting Game Engine of the bunch. Castle Game Engine has an impressively clean, simple but feature-complete Editor, it exports to many platforms, and is built on top of "fast clean code using modern Pascal" - which is also it's scripting language of choice. It has a good overview on "Why Pascal?" on it's documentation, so if it can convinces you to try it out, it might be a very fun Game Engine to build your game with.