Short answer: GBL is converted into GHB.
How?
Super easy method:
Drink it. Your body will automatically convert the GBL into GHB. It's slightly more harmful for your body than pure GHB, but not drastically so.
Somewhat easy method:
Find the conversion method on the internet, it's very easy to find, and anyone capable of making a tuna cassarole should be able to do it.