After upgrading portage tree, it seems that there is no ebuild, which could fulfill the needed requirements:

emerge  -va digikam

These are the packages that would be merged, in order:

Calculating dependencies... done!

emerge: there are no ebuilds built with USE flags to satisfy ">=dev-qt/qtgui-5.11.1:5[-gles2]".
!!! One of the following packages is required to complete your request:

• dev-qt/qtgui-5.15.2::qt-kit (Missing IUSE: gles2)
• dev-qt/qtgui-5.15.1-r1::qt-kit (Missing IUSE: gles2)
  (dependency required by "media-gfx/digikam-6.1.0::kde-kit" [ebuild])
  (dependency required by "digikam" [argument])