Activate Gwenna by tapping it, adding ({G} magic symbol) ({U} magic symbol) that can only be spent to cast creature spells or activate abilities of creatures.
Cast Uro from the command zone by paying ({1} magic symbol) ({G} magic symbol) ({U} magic symbol) .
Gwenna triggers, putting a +1/+1 counter on it and untapping itself.
Uro enters the battlefield, triggering its first and second abilities.