B m (
gnus-summary-move-article) in Gnus summary mode allows to move an article from one group to another. But how do I move not a single, but tens or hundreds of articles from one group to another? Using keyboard macros seems unidiomatic and error-prone.
What you need is process marks. Mark all articles you need to move from one group to another. For example, you can simply mark all articles you want to move by manually pressing # (
Then after you marked all needed articles, press B m and move all articles to the group you want.