Because Moshe called the Gadites <and Reubenites> “a band of sinful men” (Numbers 32:14), he was punished that his descendant worshiped Mikhah’s carved image.
The answer is that our Sages, of blessed memory, taught: Whoever lives outside the Land of Israel, it is as if he has no God (Ketuvot 110b). And our Sages, of blessed memory, said: He who suspects the innocent is stricken in his body (Shabbat 97a). Moshe <suspected them of not> wanting any portion in the Land of Israel. This implied that they had no God.
On account of this, [Moshe] was <nevertheless> punished that his descendant <would serve idolatry>. For [the Gadites and Reubenites] were blameless in this matter. They wanted to take a portion also inside the Land. As is brought <in Kiddushin>.
אִיתָא בַּמִּדְרָשׁ (הובא בספר חסידים סי' קלז, ובראשית חכמה שער ענוה פרק ה):
It is brought in the Midrash (Sefer Chassidim #137; Reishit Chokhmah, Shaar Anavah, Chapter 5):
בִּשְׁבִיל שֶׁאָמַר מֹשֶׁה לִבְנֵי גָד: תַּרְבּוּת אֲנָשִׁים חַטָּאִים, נֶעֱנַשׁ שֶׁבֶּן בְּנוֹ עָבַד לְפֶסֶל מִיכָה.
Because Moshe called the Gadites <and Reubenites> “a band of sinful men” (Numbers 32:14), he was punished that his descendant worshiped Mikhah’s carved image.
וְהַדָּבָר קָשֶׁה, מָה עִנְיָן זֶה שֶׁל תַּרְבּוּת אֲנָשִׁים חַטָּאִים, לָעֹנֶשׁ שֶׁלּוֹ שֶׁיַּעֲבֹד בֶּן בְּנוֹ עֲבוֹדָה זָרָה.
This is difficult: What does the subject of “a band of sinful men” have to do with his punishment, that his descendant would worship idolatry?
וְהַתֵּרוּץ, כִּי אָמְרוּ חֲכָמֵינוּ זִכְרוֹנָם לִבְרָכָה (כתובות קי:): כָּל הַדָּר בְּחוּץ לָאָרֶץ כְּאִלּוּ אֵין לוֹ אֱלוֹהַּ, וְאָמְרוּ חֲכָמֵינוּ זִכְרוֹנָם לִבְרָכָה (שבת צז): הַחוֹשֵׁד בִּכְשֵׁרִים לוֹקֶה בְּגוּפוֹ. וּמֹשֶׁה הָיָה חוֹשֵׁד אוֹתָם, שֶׁבְּנֵי רְאוּבֵן וּבְנֵי גָד אֵינָם רוֹצִים בְּאֶרֶץ יִשְׂרָאֵל שׁוּם חֵלֶק, נִמְצָא שֶׁאֵין לָהֶם אֱלוֹהַּ,
The answer is that our Sages, of blessed memory, taught: Whoever lives outside the Land of Israel, it is as if he has no God (Ketuvot 110b). And our Sages, of blessed memory, said: He who suspects the innocent is stricken in his body (Shabbat 97a). Moshe <suspected them of not> wanting any portion in the Land of Israel. This implied that they had no God.
בִּשְׁבִיל זֶה נֶעֱנַשׁ בְּבֶן בְּנוֹ שֶׁלֹּא הָיָה לוֹ אֱלוֹהַּ, כִּי הֵם כְּשֵׁרִים הָיוּ בְּדָבָר, כִּי רָצוּ לִטֹּל חֵלֶק בָּאָרֶץ גַּם כֵּן, כַּמּוּבָא:
On account of this, [Moshe] was <nevertheless> punished that his descendant <would serve idolatry>. For [the Gadites and Reubenites] were blameless in this matter. They wanted to take a portion also inside the Land. As is brought <in Kiddushin>.