It seems that now there is: LeafPic, see also the screenshots on the Github site (I have also posted it in Picture viewer app: what’s your choice? - I hope that’s fine). Though it may still be in (early) development phase. I haven’t test it yet, because the preinstalled open-source AOSP Gallery app is fine for me.
Btw I wonder whether the preinstalled gallery app in FPOS (with Google Apps) is actually the same as the one in FPOSOS and so is open-source as well? Could someone with a preinstalled OS (who knows how to use adb) try the following commands in a terminal after having enabled USB-Debugging? (root is not needed, I think)
$ adb shell
$ pm list packages -f | grep Gallery
The output should be one line containing the package name and path of the Gallery app. If it’s
package:/system/app/Gallery2.apk=com.android.gallery3d
this would at least be a hint that it’s the AOSP version (otherwise (i.e. if it was a proprietary Google app) it should be something like com.google.[...], I guess).