Gentile graves and cemeteries-Tumas Meis, Kohanim, Yisraeilim:
A. Laws of Tumas Meis-A Kohen coming in contact with Gentile graves, cemeteries, and corpses:
Touching the corpse of a gentile: It is forbidden for a Kohen to come into contact with the corpse of a gentile.
Tumas Ohel-Entering a cemetery and stepping on graves of gentiles: It is disputed amongst Poskim if the Tuma of Ohel applies to the corpse of a gentile. Thus, it is disputed as to whether a Kohen may enter a gentile cemetery and step on the grave of a gentile. Practically [although the main opinion follows the lenient approach] it is proper to beware not to step on their graves. [It is questionable if a Kohen may enter within four Amos of the grave or body of a gentile.]
Jewish heretic: The body of an apostate Jew has the same status as the body of a regular Jew regarding Tumas Meis, and hence a Kohen may not come into contact with his body or grave, or be under the same roofing.
Reshaim: One is not to visit the grave of a Rasha, as doing so causes Mazikim [damaging spirits] to attach to him.
Gentile cemetery: [Based on the above statement, one is not to visit the graves of gentiles.] However, a [non-Kohen] may visit the graves of [righteous] gentiles [i.e. non-idol worshipers], for the purpose of arousing one to do Teshuvah, if the graves of Jews are not available in one’s vicinity. [However, graves, and cemeteries which contain statues of idols, are certainly not to be visited. Accordingly, one is to beware not to enter a Christian cemetery which contains crosses on the graves, even for the sake of Teshuvah. Furthermore, from the teachings of the Arizal it is implied that one is to completely avoid visiting the grave of a gentile.]
A Kohen is to be stringent not to visit the gravesite of a Gentile or enter a Gentile cemetery, just as is the law regarding a Jewish gravesite. Furthermore, even a non-Kohen should be stringent not to enter the gravesite of Gentiles who were idol worshipers, such as a Christian cemetery, and based on Kabbalah should never enter the gravesite of a Gentile in any circumstance.
 See Michaber and Tur Y.D. 372:2; Beis Yosef ibid; Rambam Avel 3:3; Tumas Meis 1:12-13;
 Shach 372:4 that some Poskim rule that even according to lenient opinion it is forbidden to touch and carry a gentile corpse, and they are only lenient regarding the laws of Ohel of a gentile corpse; Beis Yosef; Aruch Hashulchan 372:5; See Gesher Hachaim 6:3 that some say this impurity is Biblical
Other opinions: Some Poskim rule that it is permitted to even touch a gentile corpse and they do not give off any impurity at all. [Yireim; See Shach ibid; Birkeiy Yosef 372; Aruch Hashulchan 372:5]
 Rishonim and sources in Talmud who rule no Tumas Ohel: Rambam Avel 3:3 and Tumas Meis 1:12-13 [brought in Tur ibid, based on Rashbi in Yevamos ibid, as explains Beis Yosef ibid] rules a gentile does not give off Tumas Ohel, and hence it is permitted to step on the graves of gentile; Yevamos 61a “Rashbi would say that the graves of gentiles do not defile in an Ohel; Bava Metzia 114a that Eliyahu was in a cemetery of gentiles
Rishonim who rule there is Tumas Ohel: Tosafus Bava Metzia 114 rules that gentiles give off Ohel impurity; Rav Shimon Ben Gamliel in Ohalos 11:9
 Tur ibid in name of Rosh Kelal 30:1 that the matter is under dispute; See Tur and Beis Yosef ibid; Birkeiy Yosef 372; Aruch Hashulchan 372:5
 Poskim who prohibit: Michaber ibid is stringent as rules Tosafus ibid, Tur ibid brings in name of his father the Rosh ibid that Rav Meir of Rothenberg was stringent and protested Kohanim who would walk
Poskim who permit: Rama ibid brings lenient opinion of Rambam ibid and Hagahos Maimanis in name of Yereim; Implication of Bava Metzia ibid and Rashbi in Yevamos ibid
 Implication of Michaber and Rama ibid; Aruch Hashulchan 372:5
 Michaber ibid; Rama “Nachon Lehachmir”; Tur ibid; Rosh ibid; Maharam; Tosafus; Conclusion of Noda Beyehuda, brought in Pischeiy Teshuvah 372:9; Kitzur SHU”A 202:10
 Chomos Yerushalayim 6, brought ion Pischeiy Teshuvah 371:13 [he however explains that certainly by a grave it is permitted, as there is no worry of touch]
 Rama 372:2; Rashba 194; Kitzur SHU”A 202:10
 Chayeh Adam 135:5; Kaf Hachaim 581:90; Nitei Gavriel Volume 2 80:9; Vetzaruch Iyun from Rama 579:3 who states one is to visit the Kevarim of gentiles if the Kevarim of Jews are not available. Perhaps, however, the Rama refers to the Kevarim of righteous gentiles. Alternatively, for purposes of arousing one in Teshuvah, one may visit even the Kevarim of Reshaim. Vetzaruch Iyun.
 Rama 579:3 regarding fast days for rain “According to this, if a Jewish cemetery is not available, then they are to visit the graves of gentiles.”; M”A 559:15 regarding Tisha Beav; Chayeh Adam 135:22; M”B 559:41; Kaf Hachaim 559:81; Nitei Gavriel 81:13
 In addition to the above sources, See M”A 579:11 “Due to the above reason of Teshuvah [explained next], it is permitted to visit the graves of gentiles”, thus implying that it is forbidden to do so under other circumstances
 See Poskim in next footnote!
 M”B 579:14; Kaf Hachaim 579:20
 See Kaf Hachaim 559:81 based on Shaar Hagilgulim p. 63 that Rav Chaim Vital once went to the Kever of a gentile, and the gentile wanted to injure him; Likewise, Tosefes Chaim on Chayeh Adam 135:51 states based on Shaar Hamitzvos that “Under no circumstances may one visit the grave of a gentile, as its considered like Doreish El Hameisim”; See Zohar Acharei Mos p. 71b that Doreish El Hameisim applies specifically when visiting the gentile idol worshipers and not when visiting a Jewish grave; Nitei Gavriel 81:13