I don't really understand the issue.
What exactly did they do?
Violation of FARA, tax evasion and other financial crimes - they made a ton of money for their job and used fake/faulty corporations and shell companies to skirt tax laws (From my understanding). Nothing linked directly to Russia or the Trump campaign, outside of the fact that dates in the indictment indicate Manafort and Gates continued their activities during and after they worked for the campaign. Gates remained close to White House officials even after leaving the campaign, well into this year.
The reveal of the third person (Papadopulous) who pled guilty relates to this though, is far more consequential to the overall investigation. The lawyers under Mueller are likely going to use him as leverage to get more information out of either Manafort and/or Gates regarding others involved.