There was a discussion that covered similar topic: Public Trust Denied - #41 by JamFed.
What is GM? What is suitably? What was the subject of the letter? You will need to elaborate on the letter.
OF-306, question no. 12, I would answer no depending on the content of the December 2018 letter. Nonetheless, both of the Standard Forms 50 indicate that you resigned voluntarily and you indicated that you did not receive any proposed or notice of adversed action (ie: Notice of Termination).
As for SF-86, you should answer accordingly. You should also truthfully annotate why you left both jobs. If I were you, I would retain a security clearance attorney before submitting SF-86.
BI interview for the FEMA position? You told the investigator everything truthfully? including reasons why you left USDA? Did you get a copy of the investigator’s report of investigation (ROI)? Did the ROI say something to the effect that you told the truth about USDA prior to being confronted?
As said, I would advise you to retain a security clearance attorney as there seems to be more to the story.