I use the envelopes that you find in the stationary section of Wally World or the Office supply store (approx 4 3/8" x 5 3/4"). They usually come in a box anywhere from 50 to 250 and I use them when I make cards.
First moisten the flap and seal the envelope. Then fold down the edges on three sides to fit the GC.
Trim the top portion using decorative scissors.
Open the envelope and make little tabs on the bottom.
I use double sided tape to stick the tabs down on the underside.
Your Gift Card is ready to slip inside.
I punch two small holes and thread some ribbon to make a handle.
You can stamp any kind of greeting on the outside for the appropriate gift.
This works great to hang on your Christmas tree.