Transmission of the frames starts while the first line of the camera is acquired. The frame will be finished and completely transferred to the ringbuffer of your system shortly after the event occures.
If you have processed a full frame (in your case the second) and the gate end event occured during processing check for a pending frame (see G2GetGrabStatus(...)). If there is a frame already delivered call ‘G2Wait(…)’ and check the metadata of this image.
Possibly you will have to implement a very short delay to be sure that the last image is delivered before checking for pending frames.
I have already asked Dalsa what the camera is doing in this special situation. As long we are waiting for a better solution you will have to live with this workaround.
regards,
Theo
Ps: @parsd wrote a little help for implementing a GigE Vision Event in 
here: GigE Vision Events