Response to complement others that already respond well.
Github is from Microsoft. As the purchase is recent we still do not know what can happen. It should not change anything for open source projects, and for private projects, if you change, it is to have free accounts as already occurs in VSTS, including using Git. Otherwise, you should have more integration, optional with other Microsoft services, without leaving aside all that exists of other products, even competitors. It's good to remember that Microsoft has partnerships with its competitors, such as Canonical and Amazon, Apple (I think it's still an important shareholder, few know of it, it was it that saved Apple from breaking in the 90s), just to name a few , if it looks good it has partnerships with everyone, nor can not have, these companies depend on each other.
I have said all of this to indicate that it has a prosperous future in improvements and sustainability. For those who do not know, Github was extraordinarily deficient and there was no indication that it could reverse that.
GitLab should suffer from the same problem as Github. If Github was unsustainable and the absurd success it was, imagine the Gitlab that offers almost everything for free. Either they break or they will be bought by an industry giant. Take this into account when using one or the other. Of course it's less risky if you use on-premisse , but you'll have to take care of it yourself. In addition, if something goes wrong, it will not do much to sue such a small company, Microsoft has a lot more at stake and needs to be much more responsible.
Today private repositories are cheaper in Gitlab, but something tells me that this will change, at least for small projects, that is, Github tends to be free under certain conditions. Just my opinion, no inside information.
I believe that Github will start to have better integrations, already have, but not native.
Time will tell what will happen. I just assure you it will not be the cataclysm you are anticipating.
The fact that a particular company does not want to say a lot, including why it can use other technologies, can do things you do not even know, that can be absolutely secondary, and can meet their demand, but not yours. Contrary to popular belief, if a large company uses well, it has a reasonable chance of not being good to you. It might be cool, but it could be a cannon to kill bird.
And before I forget, Gitlab runs within the framework of Microsoft: D
I've seen they're shifting to Google. That is, they are making engineering decisions because of marketing, to move away from a competitor (even they have it that the process started (weeks) before, the Github purchase process too, and these things are not kept secret). And between Google and Microsoft, who else is evil today ? Think well! They say at least Google did not buy them. Is not that a first step? Google paid a lot for them to go there. Do you believe it is for no reason? Gitlab is cool, but I do not understand why I trust them any more than Github.