Whether you're affiliated with Gitlab or not, it's kind of shitty to use someone's platform to push a competitor. Nothing in the terms of service forbids it of course and you're absolutely free to do so, but it just feels more ethical and grown up to extoll Gitlab's virtues by using Gitlab to host the content and link to it from HN. Putting it on Github is at least a tacit implication of its superiority along some axis, which—to my eye anyway—undermines your message.