I find a lot of contradicting sources - does anyone know the actual halachah (with sources please) ?
A lot of mishanyot seem to state that Gentiles do not contract/convey impurity (Negaim 11 and 12; Zabim 2; Nidda 10 to name a few). On the other hand, mishna Taharot 5 and Niddah 4 say the contrary.
Tosefta Zabim 2 says that while gentiles are not impure per se, they are considered thus by decree.
So what gives ?
Your first reference is about Nigee Begadim, the tsaraat of clothes. "ג,א הכול מיטמאין בנגעים, חוץ מן הגויים וגר תושב." In Negaym 3,1 The topic is 'Leprosy skin' --> the specific laws of the Torah about the dermatologic phenomenon called Tsaraat are not relevant for non-Jewish people.
"כל הבגדים מיטמאין בנגעים, חוץ משל גויים" in Negaym 11, 1 The topic is 'Leprosy of tissues'. The corresponding phenomenon of degradation of fabrics called Tsaraat Begadim has some laws in Torah, these laws are not relevant for fabrics owned by non-Jewish people.
"דם נוכרית, ודם טהרה של מצורעת--בית שמאי מטהרין; ובית הלל אומרין, כרוקה, וכמימי רגליה" in Nida 4, 3. the purpose is to decree laws with the form of the laws of Tumea. 'Tuma following rabbinic laws' but there is no Tuma following the primary Torah's law. The cause of the decrees is not our topic here.
"כל הבתים מיטמאין בנגעים, חוץ משל גויים". in Negaym 12, 1 The topic is a phenomenon of alteration of dwellings building stone called Tsaraat Batim, the laws of Tsaraat Batim does not apply to houses of Non Jewish people.
In the Mishna "גוי שראה קרי, ונתגייר--מיד הוא מיטמא בזיבה" Zavim 3, 3 There is an urologic phenomenon, a discharge from the urethra, which benefited from a special atatus of Tuma in Torah, this status (Mideorayta) is not accredited for the non-Jewish people.
The same law is in Taharot 5, 8
the impurity drives people away. There is no contradiction.
An example of simple Tuma miderabanane is in Mishna Shabbat 9, 1 "אמר רבי עקיבה, מניין לעבודה זרה, שהיא מטמאה במשא כנידה: שנאמר "תזרם כמו דווה, צא תאמר לו" (ישעיהו ל,כב)--מה הנידה מטמא במשא, אף עבודה זרה מטמא במשא"
A statue of idolatry has nothing to do with Nidda, but the Rabbis decreed that it's unclean as a Nidda. (n.b the verse is asmachta only, not a source of Torah). You understand that such a statue has no way to purify in Mikveh. Tumat goym is the same. In contrast, the Jewish Niddah can purify herself in Mikveh.
You need to know that Tuma and laws about Tuma are present and well known before Matan Torah. The Torah makes a new model of Tuma Vetahara, and cancels the others.
To make the laws more intelligible, you can read in Moreh Nevuchim III, 35; III 47
If it is not sufficient, I can perhaps add some details.
Nazir 61 A &B is a great passage in this topic. The Tuma is here Tumat Met.
אלא אמר קרא לאביו ולאמו לא יטמא במי שיש לו טומאה יצא עובד כוכבים שאין לו טומאה מנלן דלית להו טומאה דאמר קרא {במדבר יט-כ} ואיש אשר יטמא ולא יתחטא ונכרתה הנפש ההיא מתוך הקהל במי שיש לו קהל יצא זה שאין לו קהל ממאי דלמא כרת הוא דלא מיחייב אבל איטמויי מיטמו אמר קרא {במדבר יט-יט} והזה הטהור על הטמא כל שיש לו טהרה יש לו טומאה וכל שאין לו טהרה אין לו טומאה ואימא טהרה הוא דלא הויא ליה טומאה הויא ליה א''ק ואיש אשר יטמא ולא יתחטא רב אחא בר יעקב אמר שאני הכא דא''ק {ויקרא כה-מו} והתנחלתם אותם לבניכם אחריכם כל שיש לו נחלה יש לו טומאה וכל שאין לו נחלה אין לו טומאה
Surely R. Hiyya b. Abin, citing R. Johanan has said that a Gentile inherits his father in Torah-law, for there is a verse, Because I have given Mount Seir to Esau for an [inheritance]! — You must therefore mean that such as are bound to honour their fathers [are referred to]. But does it say Honour thy father in connection with nazirites? — We must therefore say that the verse, 'He shall not make himself unclean for his father or his mother' shows that only those to whom [the laws of] defilement apply [can assume naziriteship] but not gentiles to whom [the laws of] defilement do not apply. How do we know that [the laws of] defilement do not apply to them? — The verse says. But the man that shall be unclean and shall not purify himself that soul shall be cut off from the midst of the kahal [assembly], referring to such as form a kahal and excluding [gentiles] who do not form a kahal.
How does it follow [that the laws of defilement do not apply to gentiles]? Perhaps [all that is meant is that] he is not liable to kareth [excision], but [the laws of] defilement do apply [to him]? Scripture Says, And the clean person shall sprinkle upon the unclean, [teaching that] whoever can become clean, becomes unclean, and whoever cannot become clean does not become unclean.
But perhaps we may say that while [the laws of] purification do not apply to [gentiles], yet [the laws of] defilement do apply? — Scripture says, But the man that shall be unclean and shall not purify himself.
mideorayta: they can not become impure, all the time they are alive. Like living animals, or living plants (=connected to the ground). But
miderabanan: they are considered impure. This is a decret whose reason is cited in Shab. 17b. [There are a lot of examples of a touma miderabanan: the most known is the need for נטילת ידים in certain cases.]
Min hatora, only jewish body can contract touma. Probably the reason is, because impurity needs a certain degree of humanity.
A presence of a human conscience is often needed in taharot issues, as we can see in all the tractate מכשירין . And touma can happen only on a 'finished', 'complete' ustensil.
