Can i lie about unpaid work to fill gaps in my cv?

Yeah go for it everyone lies.

There was recently a tragic question on here from a young woman who was hired for her dream job, worked there for a couple weeks, and then it was discovered she had lied about something not very important on her resume. She was immediately terminated, not only losing her dream job, but also going forward with a black mark on her record.

I'd suggest you spend your energy trying to figure out a way to tell the truth instead of defaulting to making up lies.

It all depends what you are applying for, sometimes they will check and sometimes they won't, but remember if they find out you lied after taking you on they can immediately dismiss you - I'm not saying they would but they can.