Johnl's answer is saying everything: You'll have to find out if the verb you are using is using the adjective/adverb "gern(e)" or "gut".
Many sentences can be formed with both words:
Ich spiele gut/gerne Fußball.
In this case "gut" means: "well". (So you can do something well.)
And "gerne" (or "gern") means: You like to do this.
So one of the sentences means that you can play football well, and the other one means that you like to play football.
Now let's look at the first two sentences from your examples:
Mir schmeckt das Brot gut.
Das T-Shirt gefällt mir gut.
The subjects in these sentences are the bread and the T-Shirt. The bread is doing something: It tastes. And it is tasting well. But the bread does not like to do something. So "gern" cannot be used here.
Note that the word "gefallen" works differently than the word "to like" in English: Just like "to taste" the physical object is the subject in the sentence and the person who likes something is the (dative) object.
So according to English grammar the person is doing something with the T-Shirt (to like) while according to German grammar the T-Shirt is doing something (gefallen).
Now let's look at your third example:
Ich mag lieber Orangensaft.
In this example the word "gern(e)" is not used in the same way as in the examples above:
It is not an adverb which describes the verb!
In the sentence a fixed expression ("etwas gerne mögen") is used. You have to take a look into the dictionary to find out that the expression is not "etwas
gut mögen" but "etwas gerne mögen". Sorry.
And finally we can have a look at mach's example:
Ich finde Orangensaft gut.
The verb "finden" has two different meanings. In this case it means: "To think that something is somehow."
The sentence using the verb "finden" is formed with an adjective describing the object, not with an adverb describing the verb:
Ich finde den Orangensaft gut.
("Gut" describes the substantive "Orangensaft" here, not the verb "finden".)
Der Orangensaft schmeckt gut.
("Gut" describes the verb "schmecken" here.)