Who wrote the book entitled Chaar Hatsioun?
I just know the title of the book. All I know is that it is about the halakha in general.
The Sha'ar Hatziyun is footnotes to his Mishna Berura as written by Rabbi Yisroel Meir Hakohen Kagan, author of the Chafetz Chaim. Other than to reference citations, the purpose of these footnotes is to typically bring short notes of interest that are tangentially related to the core topic.
To expand on Yehuda's answer:
You are most likely referring to the Sha'ar HaTziyun (שער הציון), which is the name that Rabbi Yisrael Meir Kagan (also known as the Chafetz Chaim, after the title of his work on the laws of forbidden speech) gave to his footnotes on the Mishna B'rura (which is his halachic commentary on the Orach Chayim section of the Shulchan Aruch and the gloss of the Rama thereon).1 The following description is from the author's preface to the Mishna B'rura (my translation):
And on the bottom of the page I opened a gate named the Gate of Note (Sha'ar HaTziyun)2 in which I cited everything in the Mishna B'rura in order to know from whom these insights originated. Where I saw a difference of opinion among two acharonim (later authorities) on a matter, I was not reticent to search the corpus of works of the other acharonim to see to whom among the holy ones does their opinion incline practically - and especially in the Explication of the Gr"a of blessed memory (Bei'ur HaGr"a - the Vilna Gaon's commentary on the Shulchan Aruch), who is the light of Israel and the mainstay upon which everything depends and who is fitting to settle the matter. I also searched the books of the rishonim (medieval authorities), that I might find a matter explained explicitly or some other clear proof, for Blessed be HaShem that I have many books of the rishonim.
Aside from citation of sources, Sha'ar HaTziyun occasionally makes brief comments on the opinions of various halachic authorities. However, in-depth discussion and analysis of topics mentioned in the Mishna B'rura is generally reserved for Bei'ur Halacha, another companion commentary for the Mishna B'rura that was also authored by the Chafetz Chaim.
1 It is less likely that you are referring to a reference work by Rabbi Chaim Ya'akov Kramer of Ashmyany with the same title as above (mentioned in a comment by Shimon bM; a brief Hebrew biography of R' Kramer can be found here). His שער הציון, published in 1877, provides extensive references from the Talmud (Bavli and Y'rushalmi) and commentators for 123 topics and concepts discussed throughout the Talmud. R' Kramer's work received approbations from the Netziv and Rabbi Refael Shapiro. R' Kramer was an ardent Zionist, and most of his works included the word ציון (Tziyon - Zion) in the title. This title evokes the verse (T'hillim 87:2): אהב יהוה שערי ציון מכל משכנות יעקב - "The LORD loveth the gates of Zion more than all the dwellings of Jacob." However, R' Kramer intended to invest the title with multiple meanings. One meaning refers to the marking/documenting of sources that is the purpose of the book, as indicated by the description on the title page ("בו יצוינו כל המקומות..."; see fn. 2 below). Additionally, the title hints that the book should serve as a memorial to R' Kramer's father ("ציון לנפש מר אבא"). It is therefore unclear which pronunciation of the title R' Kramer would have preferred.
2 The name Sha'ar HaTziyun derives from the phrase "sh'arim hamtzuyanim b'halacha" (B'rachos 8a, אמר רב חסדא... אוהב ה' שערים המצויינים בהלכה יותר מבתי כנסיות ומבתי מדרשות), translated as "gateways distinguished in (or marked in) Jewish Law." (This phrase in the Talmud involves a play on words from the verse in T'hillim mentioned in fn. 1 above). The Chafetz Chaim likely chose the title as a pun, hinting at the distinction of scholarship referenced in his work, but primarily referring to the function of Sha'ar HaTziyun in marking sources of halacha.