I think this roadside sign would leave most British people nonplussed. Americans, on the other hand, would presumably find its meaning transparent.
There are two reasons why Brits wouldn’t know what to make of it:
(i) That’s not how we pronounce garage;
(ii) We’re not familiar with the expression ‘garage sale’.
To take the second point first: LDOCE defines a garage sale as ‘AmE a sale of used furniture, clothes etc from people’s houses, usually held in or near someone’s garage’. (For other American terms for this, see here.) In Britain you’d sell such items either at a ‘car boot sale’ (LDOCE: an outdoor sale where people sell things from the back of their cars) or at a ‘jumble sale’ (AmE: rummage sale). Or you could give them to a charity shop (AmE: thrift shop).
Users of LPD will know that garage was one of the words I investigated in my 1998 BrE pronunciation preference survey. This showed that younger British people increasingly prefer the thoroughly anglicized ˈɡærɪdʒ over the semi-French ˈɡærɑː(d)ʒ preferred by their elders. American-style final-stressed ɡəˈrɑː(d)ʒ averaged only 6% support.
In the BrE survey I did not separate out those who prefer a final fricative (-ʒ) from those who prefer a final affricate (-dʒ). That, however, was the focus of Yuko Shitara’s 1993 investigation of this word. She found that just over half of her American respondents preferred the fricative, just under half the affricate.
So now we see who the people are for whom the spelling “groj” makes sense. They’re the people who use final stress and a final affricate in this word, and for whom father rhymes with bother. This makes garage ɡəˈrɑːdʒ rhyme with lodge lɑːdʒ. All that is needed then is an inferred sleb-type compression (blog, 6 Dec 2011).