Could you have misunderstood? Do you realize a gallon is 4 quarts (32 oz each qt). That's a huge amount of water. How big is the pot?

The way that Calathea is double potted is too snug & can be collecting water down below &/or helping rot the roots.

I'd lift the whole inner pot out & give it a chance to drain & breathe.

Karen- no they wrote a gallon on the care card. I thought it was insane they wrote that much.

The calathea is only in one pot with drainage holes at the bottom. I provide it 1 cup of water each week

Your plants' issues, relative to the spoiled foliage, are most likely related to over-watering, a high level of dissolved solids (salts) in the soil solution, or an ammonium toxicity that develops when roots are asked to live in saturated soils. I'd suggest that you start using a 'tell' to determine when your plants actually NEED water, and only then supply it. A 5/16" wooden dowel rod, ends sharpened in a pencil sharpener, makes a good 'tell. Read more below, and please forgive the formatting. It's from something I wrote and saved.

The leaf shedding can be from a decrease in light during the last 6 weeks or so, from nutritional issues; it can be a drought response to a high level of dissolved solids in the soil solution, under-watering, or over-watering. It also appears there might be some mechanical damage (like a bruise would be to a human or animal) and some oedema symptoms on the calathea.

Using a 'tell'

Over-watering
saps vitality and is one of the most common plant assassins, so
learning to avoid it is worth the small effort. Plants make and store
their own energy source – photosynthate - (sugar/glucose).
Functioning roots need energy to drive their metabolic processes, and
in order to get it, they use oxygen to burn (oxidize) their food.
From this, we can see that terrestrial plants need air (oxygen) in
the soil to drive root function. Many off-the-shelf soils hold too
much water and not enough air to support good root health, which is a
prerequisite to a healthy plant. Watering in small sips leads to a
build-up of dissolved solids (salts) in the soil, which limits a
plant's ability to absorb water – so watering in sips simply moves
us to the other horn of a dilemma. It creates another problem that
requires resolution. Better, would be to simply adopt a soil that
drains well enough to allow watering to beyond the saturation point,
so we're flushing the soil of accumulating dissolved solids whenever
we water; this, w/o the plant being forced to pay a tax in the form
of reduced vitality, due to prolong periods of soil saturation.
Sometimes, though, that's not a course we can immediately steer,
which makes controlling how often we water a very important factor.

In
many cases, we can judge whether or not a planting needs watering by
hefting the pot. This is especially true if the pot is made from
light material, like plastic, but doesn't work (as) well when the pot
is made from heavier material, like clay, or when the size/weight of
the pot precludes grabbing it with one hand to judge its weight and
gauge the need for water.

Fingers
stuck an inch or two into the soil work ok for shallow pots, but not
for deep pots. Deep pots might have 3 or more inches of soil that
feels totally dry, while the lower several inches of the soil is 100%
saturated. Obviously, the lack of oxygen in the root zone situation
can wreak havoc with root health and cause the loss of a very
notable measure of your plant's potential. Inexpensive watering
meters don't even measure moisture levels, they measure electrical
conductivity. Clean the tip and insert it into a cup of distilled
water and witness the fact it reads 'DRY'.

One
of the most reliable methods of checking a planting's need for water
is using a 'tell'. You can use a bamboo skewer in a pinch, but a
wooden dowel rod of about 5/16” (75-85mm) would work better. They
usually come 48” (120cm) long and can usually be cut in half and
serve as a pair. Sharpen all 4 ends in a pencil sharpener and
slightly blunt the tip so it's about the diameter of the head on a
straight pin. Push the wooden tell deep into the soil. Don't worry,
it won't harm the root system. If the plant is quite root-bound, you
might need to try several places until you find one where you can
push it all the way to the pot's bottom. Leave it a few seconds, then
withdraw it and inspect the tip for moisture. For most plantings,
withhold water until the tell comes out dry or nearly so. If you see
signs of wilting, adjust the interval between waterings so drought
stress isn't a recurring issue.

You might find this helpful.

Al