Gordon's cap number for this year is $9M. [
overthecap.com]
$2M of that is prorated signing bonus, which the Broncos are responsible for if Gordon is traded, no matter when that happens.
$4.5M is salary & another $5M is in LTBE per game roster bonuses. The Rams would be responsible for that $5M cap hit if they trade for Gordon.
The remaining $2M is a roster bonus that Gordon gets if he reports to training camp in 2021. So if he reports to the Broncos' camp and then gets traded the Broncos would pay that bonus and it would count against their cap. If he's traded to the Rams before the Rams' camp opens the Rams would pay the bonus and it would count against the Rams' cap.
AlbaNY_Ram