The anime is fast catching up so the GMG arc is actually going to have to concluded in the next few months. They need approximately 6 weeks lead time to complete an anime episode from start to finish (script > planning/layout > draw cells/do music > color > voice acting > assembly/post production) , so the filler arc is planned based on the mangaka's time estimates of when an arc will be complete. (That way they don't have to pause midway in the arc for random filler episodes, or worse, pause the anime entirely.)
I have to say, they've done a good job of stretching out the arc though. Things that felt rushed in the manga got a more extensive treatment, like the Erza Monster Cleanout getting a whole episode. Yet it hasn't gone to DBZ levels of pointless stretching, and things are still moving along.