How to Recognize and Recall G Perms
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You have four cases with G Perms. G perms have headlights (a side with two adjacently permuted corners with an edge of a different color between these two corners) and a 1x2 block on another side (adjacent or opposite). The block will be in one of these four locations: F (pointing to R), RF (on R pointing to F), RB (on R pointing to B), or on B (pointing to R).
Ga - F block: R2 u R' U R' U' R u' R2 y' R' U R
Gb - RB block: R' U' R y R2 u R' U R U' R u' R2'
Gc - B Block: R2 u' R U' R U R' u R2 y R U' R'
Gd - RF Block: R U R' y' R2 u' R U' R' U R' u R2
* Notice how in Gb I showed the last R2' as the inverse to illustrate execution (the fact I move my hand in reverse to avoid a regrip).
First things first. Notice how Ga/Gb are inverse to each other, and so are Gc/Gd. If you also notice, Ga/Gc are mirrors, so are Gb/Gd. This means really you can learn one algorithm and know all four, but I suggest learning at least Ga/Gb in one day and Gc/Gd the next (but it's still quite easy to learn all four in one day).
Now, how to recognize them. First we can realize that T, R's, G's, and A's all have headlights. The T PLL has two blocks (which G's only have one) and they share a corner with the headlights. The R's have adjacent block, but they too share a corner with one of the headlights. A PLLs have the "double block" thing (two blocks that share a corner). That makes it easy to recognize G's: Look for Headlights with a single block that is not physically connected to them.
Here is where we get to recalling the correct algorithm for the specific case. The way I have done this is that I always want to push the block towards the headlights. I know that with Ga and Gc it starts out with an R2 and u/u'. How can I tell which direction? I know when the block is in back (Gc) I want to push the block towards the L side, so it's u'. I know when the block is in F (Ga), since I still want to push the block to L I do a u. These two moves should trigger the muscle memory to take over (once the algorithms are in muscle memory, that is).
We have 2 G PLLs left, and it's still just as easy. These two cases have the block on R, and it points to B or F. I like to recall the alg in this manner: The block points to either U or U', keep this in mind. I first move the R face so the block is pointing up, then I move U in the appropriate manner that the block was pointing to, and I move R back to how it was. We are now at a point again that muscle memory should take over. To help illustrate this if you have Gb you do R' U' R, and Gd you do R U R'.