For objects in singular form it doesn't matter for spoken German. With that said keep in mind that I can't give you a general rule for all cases. The sentence with the 'koffer' is different from the one Peter mentions. In the sentence 'Es ist ein grosser schwarzer Koffer, the masculine indefinite article is reffering to the word 'Koffer' where as for "es ist nicht weit" it is reffering to the gallery or the place the gallery is in. It is the common everyday German and no one would misunderstand or correct you. For written German "der, den, die, das" pronouns are replaced by "dieser, diesen, diese, dieses" or "jene, jener, jenes" and "welche, welcher, welches" instead. Although correct Germans in general avoid the pronouns 'sie, er' when referencing objects and use the articles instead.
Hope you coping well with the low temperatures