The pshat in יִשָּׂשכָ֖ר חֲמֹ֣ר גָּ֑רֶם is that Yisachar is being compared to a donkey with strong bones. GR"M = bone
Rashi: יששכר חמור גרם – חמור בעל עצמות, סובל עול התורה, כחמור חזק שמטעינין אותו למשאוי כבד.
Ibn Ezra: חמר גרם – דמהו לחמור שיש לו עצם כבד
However, the Midrash (also Nida 31a) interprets GR"M = cause.
דָּבָר אַחֵר: יִשָּׂשכָר חֲמֹר גָּרֶם – חֲמוֹר גָּרַם אוֹתוֹ, וְכִי מִנַּיִן הָיְתָה לֵאָה יוֹדַעַת שֶׁבָּא יַעֲקֹב, אֶלָּא נָהַק הַחֲמוֹר וְשָׁמְעָה קוֹלוֹ וְיָצָאת לִקְרָאתוֹ.
Leah traded dudaim for an extra night with Yaakov. She awaited Yaakov's return home, and when she heard his donkey braying signaling his arrival, she went out to greet him and take him to her tent. On that night Yisachar was conceived.
It seems strange to say that Yisachar was "caused" by the braying of the donkey. That is like saying my alarm clock going off caused me to get to shul this morning. True, I probably would still be asleep if not for the alarm clock, but the cause of my going to davening was my own decision and actions. Leah had to choose to make the effort to try to be with Yaakov for an extra night in order to conceive and produce an extra sheivet. Why give the donkey credit?
My wife thought that Chazal are trying to tell us that Leah was able to elevate even the chamor=chumriyus and use it to produce Yisachar, the sheivet that dedicated themselves to learning Torah. Could be, but i am still bothered by how that is called a gorem.