Joel presents a very negative destiny for the Gentiles.  In chapter 3, the Day of the Lord is described as the gathering of all nations in the Valley of Jehoshaphat, or Valley of Decision.  The nations will come to make war, not realizing that the Lord is actually summoning them to judgment.  The Valley of Jehoshaphat seems to be named for the incident in 2 Chr 20, where Moab, Ammon, and Mount Seir gathered in force against Judah in the days of King Jehoshaphat (reigned ca 850).  In the battle, God annihilated Israel's enemies, Israel took great plunder, and fear of the Lord came upon other nations.
    

In Joel's prophecy, the nations will be judged by one criterion -- how they have treated Israel:

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        "I will gather all nations and bring them down to the
                Valley of Jehoshaphat.
         There I will enter into judgment against them 
                concerning my inheritance, my people Israel,
                for they scattered my people among the nations
                and divided up my land" (Joel 3:2).

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          These nations cast lots for the Jews, traded boys for prostitutes and girls for wine (Joel 3:3), looted the Temple treasures, and sent the Jews of Judah into exile.  For these sins, rather than idolatry, they will be held accountable.

         

This chapter is one of the explicitly Israel-supremacist passages of the Old Testament.  Usually, God says that both Israel and the Gentiles shall be punished for their idolatry.  Here, however, He declares that those who plundered Israel shall themselves be punished.  And, contrary to Isaiah, who was busy turning swords into plowshares, Joel reverses the process:

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          Beat your plowshares into swords
                 and your pruning hooks into spears (Joel 3:10).

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After all her sufferings, Israel shall at last be vindicated.  The Day of the Lord will fulfill the Lord’s promises:

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         "Never again will I make you an object of scorn to the nations"  (Joel 2:19).

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          "Never again will My people be shamed" (Joel 2:26).

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          "Jerusalem will be holy; never again will foreigners invade her" (Joel 3:17).  

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While some nations may be annihilated or sent into captivity for good,

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          "Judah will be inhabited forever and Jerusalem through all generations" (Joel 3:20). 

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Still, His blessing and His protection are not the main privilege -- it is His Presence that He most desires to give, and only to Israel:

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           "Then you will know that I am in Israel" (Joel 2:27).

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           "Then you will know that I, the Lord your God, dwell in Zion, My holy hill" (Joel 3:17).

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His Presence will make Jerusalem holy, and will open a life-giving fountain, right in the Temple (Joel 3:18).  This accords with the visions of later prophets, Ezekiel (Eze 47) and John (Rev 22).

          

All in all, this book is not an encouragement to non-Jews.