The dictionary published by the Royal Galician Academy defines it as a "variation of the Spanish language, distinguished by the abundance of words and expressions taken from Galician language". It is Spanish with many Galician features, not the other way around.
For example, the phrase Close the window would be Pecha a ventana (Castrapo). In other languages, it would be Pecha a ventá/xanela/fiestra (Galician); Cierra la ventana (Spanish); Fecha a janela (Portuguese).
Galician reintegrationist groups, which advocate for the unity of Galician and Portuguese as a single language, also use the word Castrapo to refer disapprovingly to the current standard form of Galician, which they consider to be too influenced by Spanish and unnaturally distant from the standard Portuguese.