I used to work with GM. It was my engine of choice.
I actually love the scripting it has. For me, it is much easier to work with. And immensely versatile. I have never liked drag and drop over code.
I liked it so much as to start a tutorial series. And every so often I boot it up to help people that send me messages for it.
Yet....here I am. Construct 2 forums.
Not because of the drag and drop. Not because of the price.(ok partly because of the price) But mostly because I hate the GM level editor.
I first used C2 and honestly was lost. Felt like it couldn't do hardly anything. I was in a battle of different thought/work methods.
I'm not sure what it was, but after some time finally I understood how C2 was doing things.
I won't say I like the event system. But I will say I believe it can do most everything GM can do. And do it faster. At least on a basic level. GM on the other hand lets me easily create complex systems that are easy to change and use across many projects.
While I still greatly like GM. I can't justify using it because of the level editor. If they had a good level editor, I would probably be using it and have all its costly exporters. Native and easy exporting is amazing.
The engines aside. Because they honestly don't matter nearly as much compared to art/animation. 80% of a game can easily be dedicated to art and animating. I feel it's best to find art/animating software you love and use the engine it is most compatible with.
TLDR:
GM is a great tool. C2 is a great tool. Both can do the same things. Both have great communities.
It boils down to level editors and cost/exporters.
They work fundamentally different but can get the same results.
I can't honestly say one is better than the other.