Modulation GMSK with I/Q
As far as I know, FSK-type modulations do not use quadrature. My answer below is about quadrature, not about a specific modulation.

There are two ways to look at quadrature (I and Q) modulation: baseband, and bandpass.

In baseband, the signal is complex. You have two independent signals, each modulated using PAM, $s_I(t)$ and $s_Q(t)$. The quadrature signal is $$s_{BB}(t)=s_I(t)+js_Q(t).$$ Naturally, this signal can't physically exist. To transmit it, you need to upconvert $s_{BB}(t)$ to some carrier frequency $f_c(t)$. It becomes a bandpass signal: $$\begin{align} s(t)&amp;=\Re(s_{BB}(t)e^{j2\pi f_ct})\\ &amp;=s_I(t)\cos(2\pi f_ct)-s_Q(t)\sin(2\pi f_ct). \end{align}$$

Both signals carry the same information and are equivalent in many ways. $s_{BB}(t)$ is useful to simulate and analyze the system, because it does not have a carrier frequency. $s(t)$ is the signal that is actually, physically transmitted.