Add this in your gradle.properties file :
org.gradle.jvmargs=-Xmx2000m
From here
org.gradle.jvmargs
Specifies the jvmargs used for the daemon process. The setting is
particularly useful for tweaking memory settings. At the moment the
default settings are pretty generous with regards to memory.
edit : my answer what about the gradle daemon jvm, not the app jvm. You have to use the jvmArgs property
The extra arguments to use to launch the JVM for the process. Does not
include system properties and the minimum/maximum heap size.