I'm not sure that applies to GAM received from transfers out. I know each team receives an annual allotment of GAM and that is probably only good for the year, but I'd like to think that doesn't apply for transfer fees. I tried looking it up on the MLS Roster Rules website, but it wasn't specific.
That site will need an update fairly soon. Not only to account for new 2018 cap numbers and relative thresholds, but they also list GAM received for qualifying for CCL and then additional GAM for reaching the CCL knockout stage. But after the CCL reorganization, MLS teams already start at the knockout stage.
Will be interesting to see what some of those changes are from last year. Especially as the league has encountered various scenarios it either didn't foresee encountering or just poorly planned for (i.e., Jack being drafted in Superdraft, traded to NYCFC and sold after 2 years - is it 1/2 or 2/3 of the fee?)
For this purpose, I went ahead and copied the MLS Roster Rules from the site into a word document. Once the 2018 Roster Rules are released, I plan on doing a comparison between the two to see what updates have been made.