Decrease GPU memory on MacBook Air
Simple answer: No.

The Integrated GPU in the MBA can allocate up to 1.5 GB of system memory.

There is no way for the user to change this in macOS.

Intel HD Graphics

Apple computers using newer versions of Intel HD Graphics as the primary GPU dynamically allocate up to 1.5 GB of system memory.

Products with Intel HD Graphics 6000:


MacBook Air (11-inch, Early 2015)
MacBook Air (13-inch, Early 2015)
iMac (21.5-inch, Late 2015)


From this Apple Support document.