According to the Oxford dictionary, a warmonger is a "person who tries to cause a war or who favours war." Gaspard's first appearance in the book is him trying to anger Teagan to cause a war with Ferelden. So yes, the definition is correct,
Mmm according to that definition he would be one i suppose if only due to his efforts to attempt to initialization a conflict between Fereldan and Orlais but beyond that, it was done for a nationalistic sentiment to avert a coming even greater crisis.
So is the term accurate on the second front?
I mean he doesn't seem to "like" war, he has not said anything to the effect that he enjoys conflict, to me the character simply attempted to use it to unify his country, which as people have pointed out isn't exactly an odd notion.
So to me the term "warmonger" doesn't seem all that accurate, because to be honest either webster or oxford seem more intent on the aspect of "encouraging" a war regardless of sentiment rather on the front of actually enjoying conflict, that seems at least to me the opposite of what they should define it at as.
But then again, i am no scholar i merely have a simple understanding of conflict and human nature relating to it.