In Spanish, is it possible to make 'principal' or 'gradual' an adverb without adding -mente at the end?
An easy way to turn adjectives into adverbs without adding -mente is to replace the suffix with its actual meaning, which is "de manera" (or "de modo" or "de forma").
So if you don't want to say "gradualmente", you can say "de manera gradual". Same for "de modo principal" for "principalmente"; "de forma lenta" instead of "lentamente", etc. It's like saying "in a slow manner" instead of "slowly" in English.
Of course, you can also use synonym expressions like "poco a poco" instead of "gradualmente", "sobre todo" in place of "principalmente" and the like.
Some adjectives can be adverbialized with what today we might call the zero or null suffix, as in rápido. But there is a very small, basically fixed number of these, all of which were inherited through Latin's neuter-adjective-as-adverb formation. But neither principal nor gradual are a part of this special group of adjective-adverbs, and therefore must use -mente or other adverbializing structures (such as de forma X, de modo X, en plan X, etc.)