"... slowing gaining strength"—What part of speech is "slowing" in the phrases below?
It looks like nothing more than a typing mistake (typo) to me: "slowly" was intended, but the typist lost focus and accidentally typed the "ing" ending of the following word, instead of "ly". This is an easy kind of mistake to make when typing.
All three of the examples you cite look grammatically incorrect.
By the way, in at least one article currently accessible online, the second of your three examples has been corrected to "slowly".