To obtain a real FFT i can use Float to Complex block
?
Float to Complex will do the same thing you're seeing now
— it just writes a zero imaginary component into the stream.
If you need a signal with an actually one-sided spectrum
you can use the Hilbert block, which uses the Hilbert
transform to generate a 90° phase shifted quadrature
component. But that is just wasted compute cycles unless
your next signal processing step actually needs that result.
For viewing purposes, just ignore the other side of the
spectrum. (It would be nice if the QT GUI Frequency Sink had
an option to hide it when given float input, but as far as I
know, it doesn't.)