By forging the letter, the Arisen can obtain the gold and experience reward for the quest twice, first when given one Ser Gabrian near the entrance, and the second when given to Fedel.
Give either copy (real or forged) to either man to complete the quest.
Upon entering The Castle a short cutscene will draw attention to Ser Gabrian lurking behind a corner to the right of the entrance. This is the ONLY opportunity to speak to Gabrian.
To collect both rewards Ser Gabrian must be given his copy first (real or forged), then Fedel second (real or forged).
The game's storyline will not be altered either way the Arisen handles this quest, but for the record it would appear that Fedel is correct about signs of a rebellion, whereas Ser Gabrian is attempting a cover-up by preventing news of it from reaching the duke.
The letter is referring to the soldiers who later rebel at Windbluff Tower - as such Fedel is correct in his suspicions. Ser Gabrian is almost certainly trying to intercept the letter as a thus is in league with the treachery in at Windbluff Tower.
Once Ser Gabrian is given the letter (or a forged copy of it), the quest will be marked as "Completed". However, the Arisen can still deliver the letter to Fedel upstairs in the castle. If the current active quest does not involve him, he will have a blue question mark indicator over his head. He may speak as though the quest failed, but the "give quest item to NPC" popup menu should still appear, allowing the Arisen to turn in the letter and trigger the quest ending as usual.