That's what the GHF claims, yes. To be clear, they haven't shown any documentation showing that he was fired, while Aguilar
did show letters and messages sent by GHF hailing him as a great colleague and saying they were sad to see him go.
The only material evidence GHF has shown are small chunks of SMS conversations that are perfectly compatible with the "Aguilar was trying to convince GHF leadership to change policies" hypothesis, and WhatsApp broadcast of Aguilar telling his staff they were doing a great job.
Meanwhile, Aguilar has multiple photos and videos showing the conditions aid was distributed in which you can see right now on Youtube, testimony of seeing the GHF security contractors firing into crowds (to which the GHF replied by saying its contractors only fired above crowds, still a Geneva Convention violation), statistics that showed that people got shot during every single GHF distribution, matching testimony from Palestinian doctors and journalists and IDF whistleblowers, etc.
The evidence is overwhelming unless your curiosity cuts off as soon as you read the GHF damage control statements.