Just to clarify, between a third and a half use GitLab
in some capacity. But to answer your question, I see a lot of TFS these days. Easily 90% of the MS stack firms, and even some with more diverse stacks. And increasingly with the VS git for Windows plugin as the VCS. Those are probably around 50% of my clients.
SVN and Mercurial have a noticeable presence, say around 30% of my clients, but that number seems to be declining. I mostly see this with smaller firms with a single large, mature, legacy product that's in the cash cow stage. They're barely investing in new features, so why bother to upgrade their internal systems?
A really shocking number of firms don't have any VCS whatsoever (around 15%) or individual teams decide on their own solutions (around 10%) or individual teams may supplement the firm wide VCS for production with their own dev solutions (around 15%, which overlaps with all the other categories). A lot of those teams seem to use GitHub private repos or GitLab, though frequently I see institutional constraints prevent them from adopting a self-hosted solution (a company too dumb to set up VCS isn't likely to spring for a server--these are the same people that have dev and prod but no test servers). I frankly don't know how the firms without a VCS function. They tend to be my most challenging clients, so the answer is probably not well.
And then a really large number of firms have legacy homegrown solutions. Probably like 20%. These tend to be larger companies with an in-house tech department supporting a really unremarkable product that's been in use for 20+ years, like an eCommerce solution for a mid-sized retailer or an inventory system for a manufacturer. I think it's probably a much larger segment than shows up in a lot of stats because they tend to be... weird. They feel less like development teams and more like overgrown IT departments.
Notice that these numbers don't add up to 100. That's because there's lots of overlap, especially at larger firms that have grown by acquisition, where individual entities can operate nearly independently. Really, there's a huge selection bias here too. My clients tend to be either very well organized MS shops or nightmarishly anarchic hodgepodges. That's driven by client size and by my firm's market position and by my own sales abilities. I do better with MS shops because that's the stack I use and it makes it easier for me to speak their language, as it were. My firm targets mid-sized companies over very large companies or very small ones.
So take what I say with a grain of salt.