No--a different weight of yarn would not give you the right gauge no matter how you adjust it.

That being said sometimes doubling up the yarn will give you the same gauge. For instance 2 strands of knitting worsted can sometimes be subbed for bulky yarn but you'd have to knit a swatch to see if it would work. The gauge called for in the pattern must be adhered to to get the right dementions of the garment.

It depends, somewhat, on what you want to use instead of the specified yarn. Sometimes you might have a yarn that's a worsted, but on the lighter end, and successfully sub a heavy DK and get gauge in a fabric that you like.

The key is to do test swatches - when you get gauge, do you like the fabric you've produced? Is it appropriate for the pattern? (A flowy cardigan needs a fabric that drapes nicely, not something too stiff, for example.) Don't forget to wash your swatch as you plan to wash the finished item - I've had yarns that change a lot with washing.

Yes...you could certainly use a different yarn than on the lable....but you might need to change your needle size to get to the right stitches per inch.

If the item you are making must fit, like a sweater, then you won't get the fit. But it would not matter if you were making something like a baby blanket. It might come out just a different size but still usable. Of course, it is easy to change the size of a blanket with the number of stitches you cast on and the number of rows you knit.

If I’m knitting something I need to end up in a particular size, using different wool, I knit a swatch to see how many stitches I need to get the same size as the patter gauge, then use this to proportionally amend the number of stitches/rows in the pattern.

For instance, say on the pattern it says “10 stitches and 8 rows = 4 inches”. So I knit a swatch that is 4 inches square using the wool I have chosen. As it turns out, my swatch has 14 stitches and 12 rows.

So the pattern says, “Cast on 100 stitches.” By their gauge, it should turn out 40 inches wide (10 stitches = 4”, so 100 stitches is 10 x 10 stitches = 40”). So to get the same result, I need to cast on 14 x 10 stitches = 140 stitches.

Now, many patterns will say “knit 4” (or 15” or whatever) in stockinette/2x2 rib/garter stitch or whatever, so there’s no adjustment to make there.

If it’s a stitch pattern (fair isle, lace pattern, cable etc) you need to follow this pattern, though you may need to adjust where you start the repeats to make sure it’s centred with the new number of stitches. (Say it’s an 8 stitch repeat. In the original they’d likely have you knitting two stitches of plain at either end to centre it (8 stitches x 12 repeats =96, with 4 extra stitches so divide by two and put them on either end). But if you cast on 140 stitches, you divide 140 by 8 to get 17 repeats and 4 left over (isn’t that lucky? But if it was, say 6 you’d have three plains at other end or else the last three /first three of a pattern repeat if you want to be really spiffy).

If the pattern says, something like “decrease/increase every 4th row”, that’s effectively a change every 2”, so you’d do it every 6th row in your new wool.

It sounds complicated but really it’s just simple algebra.

I just posted a pic of my Project Linus blanket #8. I ran out of yarn and had to buy more of the green, but the green I had was a sport weight and the only one I could find to match color was worsted weight. So I cut out one of the DCs in each of the squares and it seemed to match up really well. You have to look close, and the texture is a little different, but in the pic, the worsted weight is in the lower left corner, the rest is in sport weight.

So it really depends on the project, the yarn, the needle and how tight you work...that's why it's so important to make a test gauge first before you make the actual garment.