Interesting question, because we are indeed seeing more Github results seeping into SERP's.
While Github and Gitlab aren't exactly SEO-focused (no amazing SEO tool suite), as they were not originally meant for websites that rank highly, if you write titles and copy that are well-phrased and answer the searchers' intent, you will rank.
For instance, try Googling "supplychain object standard" - an AdTech standard for bid requests and transparency in the supply chain thereof. While a relatively niche concept, it is a big deal in the AdTech world. Here is the SERP result:
The Github page ranks third. The first two are from IAB, which is a heavy hitter organization in that corner of the industry and can be very hard to outrank.
A quick search does bring up SEO functionality for Github Pages and Jekyll, but it looks to be pretty basic and manual. Which, with great content, might be all you need.
In short, I would not choose this option if you want to SEO-engineer the heck out of your site, but if that's not the primary concern, by all means.