Thread your needle with another length of conductive thread, and stitch to GND on GEMMA again. Tie a knot, then stitch over to one piece of conductive hook & loop.
This stuff can be difficult to sew through, so you may want to have a pair of pliers handy for helping to grasp the needle.
Since the hook&loop will pull at it's mating piece, stitch it very securely to the bag, around all four edges. Then tie it off and cut the tail.
Jump to the CircuitPython Code or Arduino Code section for programming your Gemma.
Now your bag should light up when it's open! If you can't see into the bag when it's shut, use an alligator clip or piece of wire to connect the two pieces of hook & loop to see if the light shut off. When you've verified the sensor is working properly, you can disconnect the USB cable and connect up a coincell battery pack for taking your project portable!