I came across the sentence:
Der Vorschlag ist gut, ich unterstütze ihn.
from a page at Linguee.
Now, will the word ihn be the same form if we are referring to a die or das word?
No. The pronoun needs to agree with the gender (and the number) of the word it refers to:
Die Idee ist gut, ich unterstütze sie.
Das Vorhaben ist gut, ich unterstütze es.
Die Vorschläge/Ideen/Vorhaben sind gut, ich unterstütze sie.
The word ihn is the masculine direct object form of "it". Neuter and feminine "it" forms do not change when they are the direct object though.
English language has completely lost grammatical genders of nouns over the centuries. Only in personal pronouns rudimentary rests of the previous Indo-European gender system survived, and together with this reduction to almost nothing also the rules changed:
In English the grammatical gender of pronouns is very strongly bound to the biological sexus. In English you refer to a male person always with masculine gender ("he"), and for female persons you always use a feminine gender ("she"). For things that are not persons, you use always neuter gender ("it"). (With some exceptions: pets, ships, etc.)
But German has kept the old Indo-European gender system, like most of the other languages that developed from this root (Spanisch, Hindi, Russian, French, etc.). Every noun has an individual grammatical gender.
Note also: The German word for girl (das Mädchen) is neuter (because Mädchen is a diminutive and all diminutives are always neuter), so when you talk about it you should use the neuter pronoun. Same for "das Weib" (crone, hag, vixen). This is the old word for a woman, but now has a pejorative connotation. But when you use this word for a lady you don't like, you have to refer to it with a neuter pronoun.
Fun fact: The german word for female/feminine is »weiblich«, but it is derived form »das Weib« which is a neuter noun (but is used for female persons).
The German word for child (das Kind) is neuter. So in german you can talk about a certain child without mentioning its biological sexus.
But since about 20 or 30 years there is coming up an exception for pronouns that refer to persons. When you refer to persons, you are allowed to use the grammatical gender for the pronoun that matches with the biologicals sexus. This allows you to refer to a Mädchen (girl) with a feminine pronoun. But this is a relatively young exception in German language, and older people still are not aware of its existence, sou you still often will hear it when someone is talking about a girl.
Also note, that the gender is a grammatical feature, that depends on the word only. It does not depend on the thing that is named with this word. If you have different nouns that can mean the same physical object, those names can have different grammatical genders: "Das Auto" (es/it), "der Wagen" (er/he) and "die Karre" (sie/she) can mean the very same shabby old limousine. But which gender you have to use to refer to this car depends on the noun that you use for it. It does not depend on the physical object.