Wow, this is a doozy. I never understood the HGSS safari zone before researching this answer. This is up there with the Yancy/Curtis trades that no one understands.
First of all, you cannot enable Gible to spawn in the SZ before the E4. This is because it requires the National Pokedex. Unless you trade the entire Regional Pokedex over to your game from another game, this isn't obtainable until post-game.
After you've obtained the National Dex and 3 gameplay hours have passed since you completed Baoba's last challenge, he will call you to tell you about blocks. Blocks are objects Baoba will unlock 6 different ones of every 3 hours, (to a max of 24,) that, when placed correctly, can alter the encounter tables and Pokemon obtainable in certain areas of the SZ.
You will need Plains Blocks and Peak Blocks, which are called Shrubbery, Red Flower, and White Flower for Plains, and Big Rock, Mossy Rock, and Puddle for Peak.
Once you've obtained blocks, use the area changing machine to make sure Rocky Beach is in the Safari Zone. Then:
You'll need to place quite a few Plains and Peak items in order for Gible to appear. A safe amount would be probably to place 13 Plains objects and 17 Peak objects, then wait 100 days so then the value of them staying there will appreciate until it reaches the amount that Gible will start appearing. It's pretty lengthy...
Yeah... Gible requires 49 Plains objects and 49 Peak objects to spawn in the Rocky Beach. However, you're only allowed to put 30 at a time in one spot. There is a feature in this mess of features that increases the value of each object over time. And when I say time I mean weeks. There is a table on Bulbapedia and Serebii that explains it.
Also:
For Gible, the values must be 49 Plains and 49 Peak, plus you must have that area active for at least 100 days. The placed blocks will multiply in value every 40 days after placement in the order Plains>Forest>Peak>Water (the next set multiplies after 10 days)
So, in conclusion, here are steps:
- Beat the E4
- Obtain the National Dex
- Answer Baoba's calls to get blocks
- Place 13 Plains blocks and 17 Peak Blocks in a Rocky Beach area
- Wait 100 real life days, game on or off, in safari zone or not.
- Gible will appear
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Hope I could help!