transposing yarn weights from one pattern to another

I have and want to use as a substitute for the Drops Big Merino yarn, Berocco Ultra Alpaca.The gauges are different. The Drops Merino gauge using size 8US needles is 17 stitches& 37 rows over 10x10cm. The Berocco gauge is 20 sts. 26rows over 10x10 cm. Pattern is in garter stitch. Does that help?