https://www.gog.com/forum/general/gog_galaxy_security_issue_cve202024574/page1

https://linustechtips.com/topic/1398157-gog-galaxy-has-a-severe-2-year-old-security-vulnerability-that-is-still-unfixed/

Galaxy like all other game clients is basically a game installer/launcher, and integrated web browser pegged to the store's website. The amount of bandwidth it uses to peruse the store catalog or other web pages integrated into the client is equal to what would be used by any other web browser to do the same thing. The bandwidth it uses to download a game is equal to the size of the game's files, which are compressed, and equivalent to downloading the game directly give or take a few megabytes due to differences in packaging, and similar to any other game platform's distribution mechanisms.

So in absolute terms, whether it takes up a small or large amount of Internet bandwidth is entirely dependent on how much bandwidth you have and subjectively what you think "too much" means for your own situation. In relative terms, it doesn't use any more bandwidth on average than any other gaming client/platform per se.

If you're concerned about bandwidth usage, disable automatic game updates in settings, and perhaps cloud storage and other features in the client that may utilize some bandwidth. Other than that I wouldn't worry about it.