Much as I personally prefer GitLab, GitHub had a bigger customer base back then than GitLab has now. GitLab may be gaining market share, but it’s mostly eating Atlassian’s portion of the market and not GitHub’s. It’s plausible GitHub undervalued themself in the sale to Microsoft of course, so a careful look at the numbers is required to evaluate the valuation, but the value comparison isn’t a great sign to me.
Separately of course, GitLab might be in line with the valuation of companies on the stock market in general which have ballooned heavily in recent years. Whether that's a bubble or permanent inflation in stock prices then becomes the relevant question to answer, I suppose.
It's worth considering that between 2018 and now, tech valuations have exploded. MSFT is up 300%. QQQ is up 100%. I doubt GitHub is valued anywhere near 7.5B today.
That's a bit of an alternate timeline.
GitLab being ~$10B 4 year later seems like similar ballbark. There is always the possibility of acquisition in the future.
(EDIT: had names reversed)