If you enter Wayward Cave through the obvious entrance that is visible beside the cycling road, you will never encounter Gible. It is not found in that part of the cave.
You may only find Gible in Wayward Cave if you enter the area through a secret entrance located underneath the cycling road. It can be difficult to locate for the first time, so here is an image to help you (credit Psypokes):
If you can position your character so that the screen appears this way and then walk upward, you will enter the secret entrance to Wayward Cave. Its structure is completely different to that of the main cave, and Flash is not required to see inside. In this cave only, you may encounter wild Gible (with 15% encounter rate in D/P/BD/SP and 20% in Platinum).
Note that in D/P and BD/SP, Strength boulders will block you from entering the cave properly. You will be unable to encounter Gible until you have your gym badge from Canalave City.
Remember to pick up the Earthquake TM at the end of the cave.
