In order to maximise spiritbonding time, always try to use HQ gear and spiritbond potions to speed things along (You won't need to use spiritbond potions for grade 1 materia farming).

Grade 1 -

Craft the following gear HQ. All materials are available from the NPC vendor in the house basement.

Hempen Coif of Gathering (WVR)
Hempen Dalmatica of Gathering (WVR)
Leather Wristguards (LTW)
Leather Shepherd's Belt (LTW)
Hempen Shepherd's Slops (WVR)
Leather Duckbills of Gathering (LTW)

Create several sets of this gear and then use it just outside of any of the main city gates (Ul'Dah, Gridania and Limsa, except the White Wolf Gate in Gridania) as a high level DPS class and they spiritbond quite quickly, plus all the animal skins you get can be sold back to the NPC.

Grade 2 -

Boarskin Choker (LTW)
Mythril Earrings (GSM)
Boarskin Wristbands of Gathering (LTW)
Toadskin Ring x2 (LTW)

Equip onto a DPS class (you could add grade 1 materia to the choker, earrings and wrists here to increase their spiritbond due to them being a higher level than the rings, 35-37 compared to the rings at 30, but it could be a better idea to make an extra set of rings; unless you're only bonding 1-2 sets) and farm in such places as Coerthas Central Highlands (Ornery Karakul), Eastern Thanalan (Gold Snurble) and I'm sure there's other places around. These will also have a chance of giving you grade 3 materia as well.

Grade 3 -

Raptorskin Choker (LTW)
Electrum Earrings (GSM)
Raptorskin Wristbands of Gathering (LTW)
Raptorskin Ring x2 (LTW)

Equip onto a DPS class (add random materia to the rings as they're higher level than the rest - not necessary)

Use a DPS class in Mor Dhona, kill any mobs here, if you want to get crafting materials, you can specifically farm the Nix, Hipppogryph, Mudpuppy, Morbol. You can also farm the Wild Hog in Urth's Fount (South Shroud) and I'm sure that you can find other places for them as well. You may get away with farming in Coerthas West but I'm not sure if the items will actually bond and your remaining gear will need to be a high level in order to compensate for the loss of stats on your right side anyway.

Grade 4 -

This one should be obtainable from the following set at a 100% rate but since I haven't fully tested it yet, can't be verified.

Dhalmelskin Bush Hat (LTW)
Dhalmelskin Vest (LTW)
Dhalmelskin Gloves (LTW)
Wyvernskin Field Belt (LTW)
Ramie Trousers of Gathering (WVR)
Dhalmelskin Fringe Boots (LTW)
Wyvernskin Choker (LTW)
Mythrite Earrings of Gathering (GSM)
Wyvernskin Wristbands (LTW)
Wyvernskin Ring x2 (LTW)

If that set doesn't give 100% drop rate, it may well still be better overall as the materials will be a) easier to make when using your best gear and b) some of the items listed in the following gearset are crafts that are listed in the master book III (marked with a *) and the books can only be obtained through collecting blue scrips. However, it should have a better chance of dropping grade 5 gathering materia due to its higher item level (once they release grade 5 materia anyway).

You should make a set of this gear (listed below) when you can as it will be with you for a little while and can be considered the best gear since the job specific gear appears to be a massive grindfest currrently (someone put it at a few months for all 3 sets of gatherer specific gear, and that's if you HQ each piece first time).

Serpentskin Bush Hat (LTW) *
Serpentskin Vest (LTW) *
Serpentskin Gloves (LTW) *
Serpentskin Belt of Gathering (LTW) *
Chimerical Felt Trousers (WVR) *
Serpentskin Fringe Boots (LTW) *
Dragonskin Choker (LTW)
Hardsilver Earrings of Gathering (GSM)
Dragonskin Wristbands (LTW)
Dragonskin Ring x2 (LTW)

Grade 5 still seems to be unobtainable at this time (except by visiting mutamix), it may well be released in 3.1 and red scrips will block most of the job specific level 60 gear as well so I will update this when I can.

If you're top end and want to be getting into the highest level gathering sooner, farming for grade 3 and 4 are your best bets for now. Happy farming :)