Learn Turkish: Evler gittikçe küçülüyor. - The houses are gradually getting smaller.

Gittikçe translates to "gradually" or "more and more." It literally comes from gitmek (to go) plus the suffix -dikçe (the more it does). You can absolutely use giderek instead (Evler giderek küçülüyor) without changing the meaning of the sentence at all.

I know the adjective küçük means small. How does it become the verb küçülüyor?

In Turkish, you can often turn an adjective into a verb meaning "to become [adjective]" by adding the suffix -l. When this suffix is added to küçük, the final -k is dropped to make pronunciation easier, creating the verb küçülmek (to shrink or to get smaller). Here, it is conjugated in the present continuous tense as küçülüyor.

Could I say Evler gittikçe küçük oluyor instead?

People would understand you, but it sounds unnatural. When talking about gradual physical changes, Turkish strongly prefers using the specific "becoming" verbs (like küçülmek for getting smaller, or büyümek for getting bigger) rather than combining the root adjective with olmak (to be/become).

Since the subject is plural (evler), why doesn't the verb have a plural suffix like küçülüyorlar?