How to fill gaps in SLC-Off Landsat 7 ETM+ Images with QGIS?

You could try the gdal_fillnodata tool which is also available in QGIS via the Raster->Analysis->Fill nodata menu. It uses an inverse distance weighting (IDW) interpolation. I just tried both that method and the single date Triangulation interpolation (in ENVI) and gdal_fillnodata looked much better. If you want to merge multiple dates, you might have to implement the localised linear histogram matching methodology in python (GDAL+Numpy) to get it to look good.