When comparing GDevApp vs GameFlow + Unity, the Slant community recommends GDevApp for most people. In the question“What are the best game engines for beginners and non-programmers?” GDevApp is ranked 4th while GameFlow + Unity is ranked 26th. The most important reason people chose GDevApp is:
Prebuilt behaviors can be added to objects. This is a very efficient way to add a physics engine or make a platformer game. Lots of behaviors are included, from the most advanced (Physics, platformer, top-down movement) to really simple ones (like the behavior to destroy objects when outside the screen or the one to drag objects with mouse or touch).
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Quickly add behaviors to objects
Prebuilt behaviors can be added to objects. This is a very efficient way to add a physics engine or make a platformer game.
Lots of behaviors are included, from the most advanced (Physics, platformer, top-down movement) to really simple ones (like the behavior to destroy objects when outside the screen or the one to drag objects with mouse or touch).
Pro Powerful events system to create games without programming
No need for coding using this system which is clear and powerful: events are composed of conditions and actions.
Actions are launched when conditions are fulfilled. This is a very beginner-friendly way of making games and is still efficient for advanced usage, contrary to most other "block"/"drag'n'drop" systems.
Pro Intuitive interface
Pro Based on GDevelop
The entire webapp is based on GDevelop, an open source native game development software available for Windows & Linux, so it benefits from its advanced development.

Pro Can download an archive of the game source
Even though the tool depends on having a server up and running, you can download a copy of your game to run locally, or host somewhere else.
Pro Great UI
Sleek and user-friendly UI.
Pro Can export your game as HTML5 and for Android
Games can be packaged for Android without relying on any third party tool. You can also export your game and download it to host it on your server or let it be hosted on GDevApp.com.
Pro Shallow learning curve
Pro Adds an abstraction layer that helps create the game logic
It is almost like talking with Unity naturally. Actions in Unity can be coded using regular English more-so than with other game engines, which makes it much easier for beginners.
Pro None of boxes and arrows, perfectly integrated into the workflow of Unity
Almost all visual scripting solutions for Unity are based on nodes, boxes and arrows, GameFlow changes this way of working incorporating actions blocks and actions using drag and drop.
Pro Excellent product
The developer supports the users quickly and with detailed responses. He is knowledgeable on the Unity API as well as developing videogames.
Pro Integrated in the Unity inspector
No windows apart, work with GameFlow from the own Unity inspector
Pro Can achieve in minutes what would take hours with classical scripting
Due to its innovative block system and without follow tutorials or look documentation will be able to create actions within five minutes in Unity.
Pro An effective advanced prototyping and scripting tool
GameFlow allows to complete a videogame in half an hour.
Cons

Con Depends on a hosting service
If the website goes down or closes down, you'll no longer be able to develop your games using this system (but you can download a backup of your game from time to time and open it with GDevelop).
Con Cannot deploy native games
For now, games developed with GDevApp can only be deployed for the Web. Android deployment is in the works, but even then, they won't be native since they are built with web technologies.
Con The documentation is not very extensive
The documentation for GameFlow could use some work. It's not very extensive and it hardly covers everything needed to know to build a game.
Con Performance could be better for 2D
Performance can be a real issue with 2D games. Unity 3D is actually a 3D game engine, and drags a lot of overhead with it.
For mobile, one could never achieve the performance of more specialized 2D engines with Unity3D.
