Can we call the relationship of Edward and Gaveston homosexual?
Yes. The main concern inthe play, is not the homosexuality; it is the diversion Gavston represents to the royalty. Edward's over involvement with Gaveston is a threat to the kingdom. Edward isn't overly interested in France's attempt to overthrow Normandy and all of the fighting in Scotland.
King Edward mailed a letter to Gaveston offering a partnership in the kingdom. Of course, in contemprary society it appears like a loving gesture of one man's love for another man. (Homosexual) However according to the royalty it is treason to say such a thing. The throne is the most valued position and fought after, so it is never "offered" or "shared".
Mortimer junior and senior promised Edward I when he was dying that they would not permit Gaveston to return. Edward the first, considered Gaveston as a "play mate" but a dangerous one, that would divert his (Edward II) attentions away from ruling England.
Remember that in the time of the nobility, they considered maintaining their ruling presence and attention to political matters as the primary goal. Anything that would get in the way was perceived as a threat, even if it was pleasurable.