Here's why:

* Base pairing rules: In DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).

* Complementary sequence: To find the complementary sequence, you simply replace each base with its partner:

* g becomes c

* c becomes g

* c becomes g

* a becomes t

* t becomes a

* t becomes a

* g becomes c

Therefore, the complementary sequence for gccattg is cggtaac.