how can i convert clothe with gimp

Yes, you'll need BodyShop.

Roughly:
1) Create a project or projects in Bodyshop.
2) Export the project, noting the location of the files.
3) Modify the bitmaps with Gimp.
4) Back in Bodyshop, test fit your changes.
5) Import the project, and give it a meaningful ToolTip!

Say you want to take the camisole from one outfit and replace the bra from a bra-and-pantie set with it...

In step 1 and 2, you'd make and export two projects, one with the camisole, one with the bra-and-panties. Decide which mesh will fit the completed project best. I'd guess the bra-and-pantie, but you might need to experiment a bit.

Open up the folder for the other project. There will be 2 or 3 files in there, the one you want is the colored one, not the black and white or the grayscale. Open the colored texture file with Gimp.

In Gimp, erase everything you don't want. You can leave a bit around the neckline and shoulders, but should get pretty close to the bottom edge of the camisole (so it doesn't overlap the panties). Now the trick... take the paint brush tool and make a tiny dot in each corner as close to the edges as you can. "Select All" and Copy.

Open up the folder for the project you're going to import. Open both the colored texture and the black and white one.

Paste the camisole onto the colored texture. Make sure the bottom doesn't overlap the panties. If it does, go back to the erasing step and try again.

The next part, you might need to experiment with because I've never tried it in Gimp. What you need to do is make the camisole area on the alpha bitmap (the black and white file) white, so the whole thing will show up, not just the bra area.

I think you'll want to paste the camisole onto the black and white file, then take the paint brush tool and paint the camisole white.

When you're done with both files, save them, but leave the files open, then go back to BodyShop. Open up the project you're going to save with; you should see the manniquin wearing your outfit.

At this point, you can make and save changes in Gimp and click the circular arrow to see them in BodyShop.

When you're happy, type a meaningful tooltip text, change the outfit catagory if you want, and import (folder with arrow to the left).

You should now be able to dress your Sim in your new outfit.