quoted from the oxford dictionary:
por.ridge /prd; AmE pr-; pr-/ (especially BrE)
(AmE usually oatmeal) noun [U] soft food made by boiling a
cereal,eg oatmeal, in water or milk
gruel /grul/ noun [U] a simple dish made by
boiling oats in milk or water, eaten especially in the past by poor
people
quoted from the food dictionary:
congee
[KON-jee]
A gruel of boiled rice and water, which serves as a background for
a host of other foods including fish, shrimp, chicken, peanuts,
sesame seed and eggs. In China, where it's also known as jook or
juk , congee is particularly popular for breakfast. In Thailand
this dish is known as khao tom gung .
assumption:
The western people first used the word "porridge" to name the
chinese food "juk". The word "congee" was later developed to give a
specific english name to "juk".(note: The word "congee" is not
recognised from formal dictionary)
conclusion:
It is correct to call all soft food made by cereal as "porridge"
including oatmeal and rice.
"Gruel" refers only to the soft food made by oatmeal.
"Congee" refers only to the soft food made by rice.