All babies go through this at one point. The formula and breastmilk is quicker to digest than baby cereal. Thus, they cannot go as long without another meal. To help baby go longer between meals, you can add a bit of cereal to the bottles... even if they do not have GERD. I did this with my first son because he had acid reflux and the flap on his tummy was not fully developed yet. I ended up doing the same thing with my second son because he has a super high metabolism. He eats like crazy... still.
I think every child has different needs, and I think mother has to gauge what their child needs. My second son was eating table food at 6 months and was fully on table food by 8 months. There was no way to satisfy his hunger with a bottle. He would eat 12 oz bottle and be hungry again in about an hour.
I also would give him cereal at night or more cereal in the bottle at night to help my boys sleep though the night... and not wake up hungry.
I think around 4 months you can start to transition to cereals anyways. Between 4-6 months I think you can try mushy foods. I used to smash up bananas, or carrots, or sweet potatoes. Sometimes I would add a bit a bit of those items to the cereal, or feed it to them alone. Sometimes I would also add it to yogurt. You can even puree items in the blender. I used to keep old baby food jars to put puree items into since I usually made enough for several meals in the blender. It was was much cheaper than baby food too. You can easily put the refilled baby food jars in the freezer and thaw out overnight, remove the lid and heat in the microwave, send to daycare-babysitter's, or leave in the fridge for a meal within the next day or two.