Summoner doesn't just have instant GCDs (whilst in Dreadwyrm Trance, when Ruin IV procs or willing to take a 20 Potency per Cast loss), most of their damage isn't even ON the GCD. A third of their damage is their own kit (which also includes oGCDs like Fester and Deathflare), a third is DoTs (including Ruination, which could be seen as a psuedo-DoT since it's 20potency added onto your Ruin spells, which you should be casting) and the final third is the pet itself (Whether that be Bahamut or the Egi).

To say that Summoner lacks mobility would be like saying Warrior needs a buff (in 4.2/4.3)