Gaining 100mil+

Majin Wasteland

There's plenty to kill in the various areas around here. There's another quest to find Ruby shards in this area (not the gem drops, these are a different variety) and once you obtain them all you can get a nice piece of arms eq with only a 50mil PL requirement. It is 850 armor for arms, no layer, 8 str 12 spd 8 spr 12 frt with 2.5% gains. If you did the lotus temple quests and the eternal dragon statue quest this should put you at 19.5% gains, which is just 0.5% shy of the max.

Feldron

Feldron is located on Hydia. You can instant transmit here to Dumastyn. Theres quite a bit to explore here with mobs ranging from 100mil to 1bil. And a lot of named mobs that will drop equipment for you if you are lucky.

Maima Region

This is probably the best gaining area for this range. You can get here by IT'ing to Maima. Mobs start around 150m and rise all the way to 2.3b. Some of the named mobs are a lot higher and you'll probably have to revisit the area to get some of their loot. You may notice getting some LUCKY messages while killing things. This area gives you a chance at better loot with more luck. If you get extras of the invasion stuff (besides the badges) you can turn them into the Namek at the 'Maima' IT point.

Capsule Corp

There's some scientists and other mobs wandering around that are in the 100-200m range. Downstairs in the basement are a TON of junkions that drop a good bit of zeni. They are androids so impossible to scan but are most likely 350m PL. There's not much to fight after these though. A few of the named mobs in the tanks drop items, but I'm not sure if any are worth it anymore. The clones range from 600m up to 5b but there's only one of each.

Zobraizure Mountains

Once upon a time when there was no other areas this was a great place to gain. Now the gaps are too large and there's not much point to explore here. The mobs start at 100m, then jump to 500m. There's even stronger ones further in. It can be an okay spot for the between powerlevels of other areas or if you're trying to get away from everyone I suppose.

Mount Areukk

There's a pretty crowded area if you head to the underside of Mount Areukk, near the entrance to the Ginyu Base you can visit the poor city for icers starting at 500mil, and there's more progressing up until almost 1bil.