Instead of using Concatenate
mode you can remove the gaps using the option
-geometry '1x1+0+0<'
The geometry flag specifies the perferred tile and border size. The geometry of 1x1+0+0<
tells it to remove the border (the 0+0
part) and to enlarge images smaller than 1x1 (the 1x1<
part).
The 1x1
resize is necessary since if you use a 'zero geometry' it will automatically put it into concatenate mode (see here).