Env vars differ in first and second terminal
I haven't tried to reproduce the exact same problem with ssh-agent. The problem over there, as far as I understand, is that second etc. tabs or windows (ones opened from within gnome-terminal, rather than from an external request) have fewer env vars: SSH_AUTH_SOCK and GPG_AGENT_INFO are missing.
What I see is that second and subsequent tabs (ones opened from within gnome-terminal, e.g. Ctrl+T, Ctrl+N) have more environment variables, e.g. G_ENABLE_DIAGNOSTICS, SESSION_MANAGER, UNITY_DEFAULT_PROFILE, UNITY_HAS_3D_SUPPORT, XDG_MENU_PREFIX, ZEITGEIST_DATA_PATH, and a different value of GTK_MODULES.