That PR/MR icon is quite terrible, I much prefer github's which actually communicates the idea clearly and abstractly and is uniquely recognisable. This other one is just two squares, it could really mean anything.
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.
I love Gitlab features, but the down itme is really a downer for us. Used the self hosted free version at previous work for 2 years, was sufficient for most use cases for 50 people.