In Volume 1 of his high-quality series on the Grünfeld for black, GM Avrukh recommends 3....b5!?, "playing in the spirit of the Benko Gambit". On page 6 of the PDF excerpt, you can find his analysis.
However, this may not be to everyone's taste. Instead, he also gives the line 3....c6 4.Nc3 cxd5 5.cxd5 d6 6.e4 Bg7 etc... "with equal chances". To me, this seems a good way to play: black has several ways to develop his pieces on the queen side and to create counterplay (e.g. Nbd7-c5, e6) and it is not necessary to memorize any complicated lines.
With 3.d5 white indeed avoids any Grünfeld structures, but it is relatively harmless and black can develop quite harmoniously.
1.d4 Nf6 2.c4 g6 3.d5 c6 (3...b5) 4.Nc3 cxd5 5.cxd5 d6 6.e4 Bg7 7.Nf3 O-O 8.Be2 Nbd7 9.Be3 Nc5 10.Nd2 e6