Using the ! character before the path to the file should work, in your .gitignore file

# Ignore all .jar files...
*.jar

# ...except gradle-wrapper.jar
!path/to/gradle-wrapper.jar

From https://git-scm.com/book/en/v2/Git-Basics-Recording-Changes-to-the-Repository#%5C_ignoring:

You can negate a pattern by starting it with an exclamation point (!).