Personal Data is any Data that either on its own or when cross-referenced with other Data allows the univocal Identification of a Natural Person. So for the company towards which you register to get your "steam Account ID", YES it is Personal Data, for they can identify you through it; it is also Personal Data for any partner they share such identifier with and/ or players towards whom the same happens... it is not Personal Data however for those who only can "see" your "Steam Account ID".
If Valve shares your Steam Account ID with me and the score that you have achieved over several games, as long as there is no way for me to identify you (as a Natural Person) via such Data... it is not Personal Data, hence GDPR does not apply. However that would mean that Valve does not share your Steam Account ID with any other 3rd party whatsoever, for if it does, at least that 3rd party may identify you as the owner fo that Steam Account ID and them all inherent profiling Data becomes Personal Data... in some cases it is a matter of Context, really.
If they have shared your Steam Account ID with other companies or entities where some other 3rd parties can use it to univocally identify you, making it Personal Data and it was shared without your awareness and a Lawful Base (Consent; Legitimate Interest; Legal Obligation; other...) what you should do is:
Step 1 - Submit a DSAR (Data Subject Access Request) inquiring about which Data they have about you; the Lawful Base and with which other entities do they share your Personal Data with under which Lawful Base.
Step 2 - If no answer within 1 month or "NO" for an answer, present a complaint towards the local Supervisory Authority; If there is a comprehensive answer and they tell you they share the Data with Company A... (n), you may then on«object to such sharing unless they have explained HOW and WHY without it they are not capable of delivering the Service.
In the U.S. Personal Data (in fact PErsonal Individual Information) is both a far less broaden context as it is not up to resident Natural Persons to exercise the same types of "rights" that GDPR defines.
As an example under the California, Customer Protection Act (similar to GDPR, yet only for residents of the State of California) companies may pretty much do as they please with your Personal Data until you "opt-out".
So, making a long story short, EU resident Natural Persons have control over their Personal Data as long as they exercise their GDPR established rights, whereas in comparison Natural Persons residing in the U.S. have a much limited set of rights they may exercise.
This is a "broader answer" to your question, meaning adding some details (e.g. your Id shared amongst the gamers of the same gaming group on the platform) may render the entire logic "void".
These platforms/ forums that are either "free" or bear a very "low fee" are businesses, not charities, and that means they need to be profitable, so at the end of the day, some will either charge nothing or a very low fee and compensate for that by selling your Profiling Data to companies that may then attempt at selling you something... it all would be fine if you were informed about such proceedings in advance in a crystal manner having Consented to it... however, that is not how these companies act and that puts them in breach of GDPR.