Companies need to make money. Not billions of dollars in profits like Google, Amazon, Tesla, etc. But they need to at least make money to keep their services up, and then pay their employees decent wages. Otherwise they literally go bankrupt.
Software is unique in that it basically costs $0 to distribute infinity copies. Sure you may need to periodically update and host your software, but you don't really need to do that and it usually basically costs $0 to update and host anyways. As a result, most software is free! We have entire operating systems, IDEs, professional-grade tools (e.g. Blender), game engines all for free, because they can be maintained on low budgets and people's spare time. And this is how it should be, software should be free.
But this leads lots of developers to expect that everything should be free, when this is flat-out impossible. You can't expect your food, heating, clothes, etc. to be free because they cost physical resources to produce. You can't expect Google to sell laptops for free. Similarly, you can't expect hosting providers to sell storage for free, because it actually does cost resources to run those servers. GitLab has to pay for more hard drives, maintenance, electricity, rent, etc. as more people use their service, and as people use more of their services.
Blender doesn't really have to pay more whether 1,000 or 1,000,000 people use it, GitLab does. Linux doesn't really have to pay every month to keep their kernel online, GitLab does.
And yeah, companies like GitLab have paid tiers. The issue is that almost nobody uses them, even when they really should. People love to buy "free" stuff, even if something costs $1 you will have far less people buy it. Hence, why we have this entire culture of companies "buying" you via ads and tracking. They do it because consumers encourage ads and tracking in their spending habits,
And yeah, companies do have marketing. Another thing software developers don't like to accept is that companies need to market, you can have the best product but without marketing you will sell it to noone. Marketing doesn't mean spammy advertisements and aggressive tracking, there are good ways to advertise your product (like posting an in-depth review to HN with benefits and drawbacks). And marketing absolutely doesn't count as "payment". That attitude is like those influencers asking for services in exchange for "you'll be paid in reputation and experience"!
You're absolutely right that nobody needs to use GitLab, and if nobody uses it GitLab will shut down. If all developers team up and decide that they won't use GitLab unless they offer everything free, then they will shut down. Do you want that to happen? Boycotts are good when you're boycotting something like Battlefront 2 for ridiculous micro-transactions, where EA actually has the resources to scale back (and if they don't, it is they're fault, because they're charging $60 for the base game), and a competitor can form and take over. But if you boycott companies with reasonable policies for making anything paid whatsoever, you're going to end up with 0 hosting providers, no competitor is going to appear, because they simply can't exist without going broke.