I read the last meeting log and I can perhaps add some helpful info. The tools consists of two parts, a host daemon and a client loader. The host daemon configures your system when the client asks it to. The game can contain the client loader natively (out of the box functionality, Feral games), or you can use the provided libgamemodeauto library with LD_PRELOAD together with any game.
The tool contains a Type=dbus systemd service:

$ cat /usr/lib/systemd/user/gamemoded.service
[Unit]
Description=gamemoded

[Service]
Type=dbus
BusName=com.feralinteractive.GameMode
NotifyAccess=main
ExecStart=/usr/bin/gamemoded -l

[Install]
WantedBy=default.target

and a dbus service itself:

$ cat /usr/share/dbus-1/services/com.feralinteractive.GameMode.service
[D-BUS Service]
Name=com.feralinteractive.GameMode
Exec=/usr/bin/gamemoded -d
SystemdService=gamemoded.service

I'm a layman, but it looks to me like the host daemon is triggered by the dbus service. Therefore I think it could be possible to make use of this tool even from a flatpak'd game (if it exposes dbus), even without creating additional portals.

I hope this helps.

Since this ticket got approved today, here's a PR for comps (please either merge or approve, thanks):
https://pagure.io/fedora-comps/pull-request/373

From today's meeting:

14:22:56 <kalev> #agreed Workstation WG agrees to install gamemode by default. While we think the design is fundamentally not ideal, the security implications of allowing the current session to adjust cpu governor and gpu clocking are small.

https://meetbot.fedoraproject.org/fedora-meeting-2/2019-02-25/workstation.2019-02-25-14.01.log.html

Commit 35d36cf9 relates to this ticket

Fixed commit link (a bug in pagure):
https://pagure.io/fedora-comps/c/35d36cf9

We have 2 system developers, including one of the founding members of the Games SIG saying that this needs to be reverted.

See:
https://lists.fedoraproject.org/archives/list/desktop@lists.fedoraproject.org/thread/33F4YP7EYNLKKTC57NEQWDHBN4Z5SVCG/

This is just one of many past examples where we decided to install something new for fresh installs, but never provided a way to make upgrades work (@chrismurphy)

This is in the news currently because Ubuntu is following us in installing it by default.

BTW, we have it vendor preset to disabled. Is it expected that games are required to manually start and disable the service? Or that users are required to manually do so (in which case it's not a good default)?

BTW, we have it vendor preset to disabled. Is it expected that games are required to manually start and disable the service? Or that users are required to manually do so (in which case it's not a good default)?

As I understand it, it activates automatically over dbus. You can try it yourself using gamemoderun glxgears and then systemctl status --user gamemoded.

This is installed by default on Fedora? I'm not seeing it.
$ systemctl list-unit-files | grep game

Try: systemctl --user list-unit-files | grep game

DOH! Yes, it's there and disabled.

Reopening. We should have a Recommends or Supplements somewhere to pull this in for upgrades.

Maybe this issue, #60 and #66, should set to depend on #138. And discuss #138 at a future meeting?

Having too many open issues is getting messy. This should be fixed soon in #138.