Quomodo dicere "They left to eat ice cream"

[Quis ut Deus]

Salvete amici amicaeque!

Valeo si valent.

Quomodo dicere "They left to eat ice cream?"

Scribo sic sententia mea:

"Discesserunt gelati edendi causa." (GERUNDIVE, anglice)

vel:

"Discesserunt ut gelatum ederent."

Gratias vobis ago!

Valete

[adrianus]

Both are good, I believe, Quis ut Deus: your genitive gerundive with "causâ" or the one using the "ut" clause.
Uter modus, Quis ut Deus, bonus est, ut credo: iste per formulam cum gerundivo casu genetivo vel iste cum ut conjunctione.

[Quis ut Deus]

Salve Adriane!

Multas Gratias tibi ago!

[furrykef]

You can also use the gerundive with "ad" (here I'm guessing the word is "gelātum"):
Discessērunt ad gelāta edenda.

And since this is a motion verb, you can even use the supine:
Discessērunt ēsum gelāta.

While the gerundive with "causā" is surely a valid construction, I'm not sure it is the best one. I understand "[gen. noun] + causā" to basically mean "for the sake of [noun]", and saying "They left for the sake of ice cream" sounds a bit odd. Wheelock doesn't use this construction where "for the sake of" would be awkward in English. Of course, that doesn't mean that the Romans didn't...

Where did you find the Latin word for ice cream, BTW? Or did you model it on Italian "gelato"?

[Quis ut Deus]

@furrykef-Thanks for the input!

Yeah, I started with "helado" in Spanish, and then went with "gelatus" in Latin.

[adrianus]

Traupman (Conversational Latin for Oral Proficiency) gives "gelidum crēmum" for icecream.
In Morgan, these (including your "gelatum"):

.swet ice cream / Eiscreme: glacies (edibilis) -- Speiseeis: glacies edibilis; gelatum [Vox Lat.] (HELF.)
.swet ice cream / gelida sorbitio, sorbillum glaciatum (LRL); glacies edibilis (ALB. I)
.swet ice cream nix, cremor lactis gelu concretus (LEV.)

Pro anglicè "icecream" apud Traupman "gelidum crēmum" habes. Apud Morgan haec citata (quae gelatum tuum includunt).

"They left for the sake of ice cream" sounds a bit odd

Don't say that in English, then. Say "for the purpose of eating" or "to eat". See A&G §504b.
Sic anglicè non dicas sed aliter. Vide A&G sectionem quingentesimam quartam partem b.