Cant trim a gap clip when using dissolves??

I'm dissolving into and out of a gap clip for effect, yet when I try to make the gap clip smaller or bigger by using the crtl D function and typing in the new length of the clip, a lot of the time nothing happens, I'm not allowed to use the trimmer tool either as it only seems to let me trim the dissolves rather than the gap clip.
Any ideas why this is happening?
I know there is another effect in the transitions section that will create the same effect I'm looking for but I cant for the life of me remember what it is, anybody?
Thanks
S

The ripple and roll controls to trim under the transition are on the bar at the top of the transition, ripple controls on the ends and roll in the middle.

Do you mean Fade to Color (under Dissolves)???

No. I mean any transition.

Sorry, I was answering the OP's question as to whether there was another transition that might achieve what he's looking for without using gap clips.