The Celestial Dragon summoned the marines to God Valley after Rocks Pirate attacked the same way they did in Sabaody Island?
The admirals are obliged to go, but vice admirals are subordinated to the admirals. Garp was probably "subordinated" to Sengoku, who would turn a blind eye to him as he probably did several times in the past. On top of that, God Valley was in the West Blue and Garp was a HQ vice admiral, something dread must've been happening for him to be there.
One possible explanation may be that Sengoku wasn't an admiral yet and/or another admiral (Garp's superior) was escorting the tenryuubitos in the Native Hunting Competition. That would explain Garp's involvement and Sengoku's absence (he wasn't mentioned in the God Valley Incident so far, so that's a possibility).