To the original problem, there is a way to ignore the outdated GPG sig. please go through these steps:
As egrath mentioned, winepak is quite outdated, not sure which game you are looking for, so I pull the current list from winepak for you to save some time:
BaseApp com.blizzard.BattleNet.BaseApp stable
Overwatch com.blizzard.Overwatch stable
StarCraft2 com.blizzard.StarCraft2 stable
WoW com.blizzard.WoW stable
Client com.leagueoflegends.Client stable
Planet com.oskarstalberg.Planet stable
Client com.pathofexile.Client stable
Client com.worldoftanks.Client stable
Cemu info.cemu.Cemu stable
Notepad-plus-plus org.notepad_plus_plus.Notepad-plus-plus stable
Compat32 org.winepak.Platform.Compat32 3.0
corefonts org.winepak.Platform.Extension.corefonts 3.0
d3dx9 org.winepak.Platform.Extension.d3dx9 3.0
msls31 org.winepak.Platform.Extension.msls31 3.0
vcrun2010 org.winepak.Platform.Extension.vcrun2010 3.0
vcrun2012 org.winepak.Platform.Extension.vcrun2012 3.0
vcrun2013 org.winepak.Platform.Extension.vcrun2013 3.0
vcrun2015 org.winepak.Platform.Extension.vcrun2015 3.0
vcrun2017 org.winepak.Platform.Extension.vcrun2017 3.0
Compat32 org.winepak.Platform.Wine.Compat32 3.10-staging
Compat32 org.winepak.Platform.Wine.Compat32 3.8-staging
Compat32 org.winepak.Platform.Wine.Compat32 3.9-staging
Wine org.winepak.Platform.Wine 3.10-staging
Wine org.winepak.Platform.Wine 3.8-staging
Wine org.winepak.Platform.Wine 3.9-staging
Platform org.winepak.Platform 3.0
Sdk org.winepak.Sdk 3.0
For example, if you want to install StarCraft2, make sure uses root permission to bypass the signature check:
$ sudo flatpak install winepak StarCraft2
HTH