Bayern announced Saturday that Gnabry tore the adductor muscle in his right thigh and will be out "for a longer period," putting his participation in the finals in doubt.
The 30-year-old has now confirmed he'll be forced to watch the tournament from home.
"The last few days have been tough to process," he posted on Instagram on Wednesday. "A Bayern season which still holds much to play for after securing another Bundesliga title on the weekend.
Serge Gnabry has been a key player for Germany and Bayern Munich. Angel Martinez/Getty Images)
"As for the World Cup dream with Germany .. That's sadly over for me. Like the rest of the country I'll be supporting the boys from home.
"Now it's time to focus on recovery and getting back for pre-season."
The former Arsenal player started every World Cup qualifier for Germany and two friendlies in March, and was sure to have been included in coach Julian Nagelsmann's plans for the tournament in Mexico, Canada and the United States.
After clinching the Bundesliga title on Sunday, Bayern face Bayer Leverkusen away in the German Cup semifinals on Wednesday, before they turn their attention to Paris Saint-Germain in the Champions League semifinals.
Gnabry contributed eight goals and set up seven more as Bayern broke the Bundesliga record for goals scored.