Al-Haashir (the Gatherer) is one of the names of the Prophet (peace and blessings of Allaah be upon him); it is not one of the names of Allaah
Question

                One of the names of the Prophet (peace and blessings of Allaah be upon him) is al-Haashir (the Gatherer) because the Prophet (peace and blessings of Allaah be upon him) said: “I am the Gatherer at whose feet the people will gather” or words to that effect. We hope that you can explain this hadeeth. I know that Allaah is called al-Haashir and He is the One Who will gather the people for the Reckoning on the Day of Resurrection. I hope that you can clarify this matter. May Allaah reward you with good and place this in balance of your good deeds.
Praise be to Allah.
    This hadeeth – “I am the Gatherer at whose feet the people 
    will gather” – is one upon which the two Shaykhs (al-Bukhaari and Muslim) 
    are agreed. The hadeeth in full is as follows: The Messenger of Allaah  
    (peace and blessings of Allaah be upon him) said: “I have several names. I 
    am Muhammad and I am Ahmad and I am al-Maahi (the Eraser) by means of whom 
    Allaah erases (yamhu) kufr, and I am al-Haashir (the Gatherer) at whose feet 
    the people will gather, and I am al-‘Aaqib (the successor, i.e., the one who 
    succeeds the other Prophets in bringing about good, or the Last Prophet).” 
    Narrated by al-Bukhaari, 4896; Muslim, 2354. 

    With regard to the phrase “I am the Gatherer at whose feet 
    the people will gather”, what this means is that they will be gathered after 
    him, and he will be gathered before the people. This is in accordance with 
    what he said in another hadeeth: “The people will be gathered at my heels.” 
     

    It is possible that what is referred to by feet (qadam) is 
    time, i.e., my era when my mission starts, when the signs of the gathering 
    begin to appear, referring to the fact that there will be no Prophet after 
    him. This was suggested by al-Haafiz in al-Fath, 6/557.

    Ibn al-Qayyim said in Zaad al-Ma’aad (1/94): It is as 
    if he was sent to gather the people. 

    Secondly: Your saying that Allaah is also called al-haashir 
    is not correct, because the names of Allaah are determined by the texts of 
    the Qur’aan and Sunnah, and there is no room for human reasoning with regard 
    to them. We must accept what has been narrated in the Qur’aan and Sunnah and 
    not add anything or take anything away. 

    1 – Because human reason cannot encompass the names that 
    Allaah deserves, so we must accept what the texts say, because Allaah says 
    (interpretation of the meaning): 
“And follow not (O man, 
    i.e., say not, or do not, or witness not) that of which you have no 
    knowledge. Verily, the hearing, and the sight, and the heart of each of 
    those ones will be questioned (by Allaah)”

    [al-Isra’ 17:36] 

    “Say (O Muhammad): (But) the things that my Lord has 
    indeed forbidden are Al‑Fawaahish (great evil sins and every kind of 
    unlawful sexual intercourse) whether committed openly or secretly, sins (of 
    all kinds), unrighteous oppression, joining partners (in worship) with 
    Allaah for which He has given no authority, and saying things about Allaah 
    of which you have no knowledge”

    [al-A’raaf 7:33]

    2 – Because calling Him by a name by which He has not called 
    Himself, or denying one of the names by which He has called Himself is a 
    transgression against His rights. We must observe proper etiquette in this 
    matter and limit ourselves to those names that have been mentioned in the 
    texts. 

    3 – The attributes of Allaah are greater than His names, 
    because each name refers only to one attribute. For example, one of His 
    names is al-Samee’ (the All-Hearing). This name affirms that al-Samee’ is a 
    name of Allaah and affirms that He possesses the attribute of hearing. 

    With regard to the actions that are attributed to Allaah, 
    they do not include the names of Allaah, because they are connected to His 
    actions, and His actions have no end (or limit). Among the actions that are 
    attributed to Allaah are coming, taking, withholding and striking, as He 
    says:  

    “And your Lord comes with the angels in rows”

    [al-Fajr 89:22] 

    “Do they then wait for anything other than that Allaah 
    should come to them in the shadows of the clouds and the angels?”

    [al-Baqarah 2:210] 

    “So Allaah seized (destroyed) them for their sins”

    [Aal ‘Imraan 3:11] 

    “He withholds the heaven from falling on the earth except 
    by His Leave”
    [al-Hajj 22:65] 
“Verily, (O Muhammad) the 
    Seizure (punishment) of your Lord is severe and painful”

    [al-Burooj 85:12]

    These things are attributed to Allaah in the way in which 
    they are mentioned in the Qur’aan, but we do not call Him by them, so we do 
    not say that among his names are “the Comer” or “the Seizer” or “the 
    Withholder” etc, but we say about Him and attribute that (action) to Him. 

    See Risaalat Qawaa’id al-Mathlaa by Shaykh Ibn 
    ‘Uthaymeen (may Allaah have mercy on him), 2/283, in Majmoo’ Fataawa wa 
    Rasaa’il al-Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on 
    him).