Later, severely depleted South Africa drew 0-0 with Zambia in a lively second match of a double-header at the Al Salam Stadium.
Gnaka scored after Olisa Ndah committed a foul that resulted in him being red-carded, leaving Nigeria to play the final 20 minutes a man short.
Nigeria lacked captain Azubuike Okechukwu and Taiwo Awoniyi after their Turkish and German clubs refused to release them ahead of the November 11-19 international window.
Many European clubs did agreed to free players for the tournament, which doubles as the Africa U23 Cup of Nations, even though it started and will finish outside the window.
Meanwhile, South Africa and Zambia provided plenty of free-flowing football only to be let down by poor finishing.
South Africa arrived in Egypt with just 12 players, and lacked six of the 21-man squad when they faced Zambia due to some domestic clubs taking a hard line on releasing players.
Former South Africa great Neil Tovey was furious with the clubs, saying: "It is a disgrace. It is downright diabolical that they cannot have the best players."
Tovey, who captained hosts Bafana Bafana (The Boys) to victory in the 1996 Africa Cup of Nations, is now the South African Football Association technical director.
On Friday, hosts Egypt edged Mali 1-0 and Ghana came from behind to draw 1-1 with Cameroon in the first series of Group A matches.
The finalists and the winners of the third-place play-off will represent Africa at the 16-nation Tokyo Olympics football tournament.
Nigeria have won gold and silver medals, Cameroon gold and Ghana bronze at previous Games.
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