documentation/docs/gentoo-desktop-setup/graphical-session/login-manager.md

River is a wayland tiling window manager which is configured only using its riverctl interface. It is rather minimal but still remains functional. River is coded using Zig but this is by default masked on Gentoo so first unmask them:

dev-lang/zig ~amd64
app-eselect/eselect-zig ~amd64

And login manager is a small graphical interface providing a nice login screen for getting into a graphical session. In this guide it is assumed that River will be used as window manager and as such it will also be used as the backend for the login manager. Before emerging the necessary packages first add some USE flags for river and keywords for gtkgreet:

gui-wm/river X
gui-libs/wlroots X
x11-libs/libxkbcommon X
media-libs/libepoxy X
media-libs/libglvnd X
media-libs/mesa X

gui-apps/gtkgreet ~amd64

Then emerge River and the greetd components:

sh# emerge -av river gtkgreet greetd display-manager-init swaybg

Now greetd has to be configured to launch River with gtkgreet. Besides that styling gtkgreet can be done using css files and GTK themes and using this configuration a wallpaper which resides at /etc/greetd/current_wallpaper.png.

[terminal]
vt = 7

[default_session]
command = "river -c /etc/greetd/river.init -no-xwayland"
user = "greetd"

#!/bin/sh

riverctl spawn "swaybg -m fill -i /etc/greetd/current_wallpaper.png"
riverctl spawn "GTK_THEME='Orchis-Dark-Compact' gtkgreet -l -s /etc/greetd/gtkgreet.css; riverctl exit"

Make sure /etc/greetd/river.init is executable!

@import url("/etc/greetd/wal/colors-waybar.css");

* { 
	color: @foreground;
	background-color: transparent;
	font-family: "Roboto";
}

box#body {
	padding: 10px;
	background-color: @background;
	border-radius: 2px;
}

dbus-run-session -- river
bash

@define-color foreground #c3c4c7;
@define-color background #121320;
@define-color cursor #c3c4c7;

@define-color color0 #121320;
@define-color color1 #5D629A;
@define-color color2 #6E729D;
@define-color color3 #7E82A9;
@define-color color4 #9698AB;
@define-color color5 #9195BC;
@define-color color6 #9EA1C0;
@define-color color7 #c3c4c7;
@define-color color8 #5e6073;
@define-color color9 #5D629A;
@define-color color10 #6E729D;
@define-color color11 #7E82A9;
@define-color color12 #9698AB;
@define-color color13 #9195BC;
@define-color color14 #9EA1C0;
@define-color color15 #c3c4c7;

Finally configure display-manager to start greetd upon boot:

CHECKVT=7
DISPLAYMANAGER="greetd"

Then enable the display-manager service to start on boot:

sh# rc-update add display-manager default

Before enabling the display-manager service it is recommend to simply first start it ones to see if it works.