Do you have the plugin BIMP for GIMP?
If so, File > Batch image Manipulation...
1 then select "Other GIMP Procedure", then 2 to search
then click 3 then 4, it seems to be a bug, you can't access 4 if you don't hit 3 before
Play with the saturation
Now as wonderful as is this plugin, if you have 50, one hundred or more images I would recommend to not use GIMP, but XnConvert which is a software dedicated for super massive batch processing photos way faster, especially if it's just about simple operation like saturation.
If so, File > Batch image Manipulation...
1 then select "Other GIMP Procedure", then 2 to search
then click 3 then 4, it seems to be a bug, you can't access 4 if you don't hit 3 before
Play with the saturation
Now as wonderful as is this plugin, if you have 50, one hundred or more images I would recommend to not use GIMP, but XnConvert which is a software dedicated for super massive batch processing photos way faster, especially if it's just about simple operation like saturation.