Daniil,
MyEclipse 2015 is shipped with JDK 1.7, but you can point it to use an external JDK 1.8. Go to Window > Preferences > Java > Installed JREs and add the new JDK.
If you are still seeing issues, let us know what exact version of JDK you are looking for along with the details about how you are using MyEclipse with Gradle system.
Let us know how it works for you.