How can you get 6 gems in a row in Bejeweled Twist?

You'd have to set up a cascade to be able to get longer lines of gems. You will not be able to do this in a single move. (Disclaimer: I don't have this version of the Bejeweled but the concept is the same in any game like this)

Here's one such cascade you can make which should be relatively easy to set up.

Try to get your gems in this pattern:

  AA
  BB
  CC C
  BB
AABBAA

Then to finish it off, just move gem C in place to start the cascade.

Getting off longer lines are often difficult to set up. Here's another variation for a 7/8 gem line.

  A  A
  B  B
 CCD DD
  BC B
AABAABAA

Then rotate C and D in place to set off the cascade.

You'll get the idea.