Geometric representation of GMSK signal
First, let's talk about MSK. MSK, just like 2-FSK, have 2 symbols, therefore, the baseband signal for both of them is no different, just a string of one-bit values. However, the passband signal differs in a way that when the frequency changes (resulting from a change of bit value), it does not involves abrupt phase changes. If you are familiar with a DDS, imagine it as changing the frequency tuning word for the phase accumulator without resetting the phase accumulator to 0 first, but letting the phase accumulator continue on its current value, just in a faster or slower way, depending on its frequency tuning word.

Now, similar to MSK, GMSK also does not involves abrupt phase changes in the passband, but, different to both FSK and MSK, its baseband signal is Gaussian filtered first, therefore, it does not look like a regular string of ones and zeros anymore, but a smoothed out string of ones and zeros. Therefore, in the passband, frequency changes are smoothed out. Here is a picture of a GMSK baseband signal with BT = 0.4.

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As you can see, even though it is essentially a 2 symbol modulation, the "ones" and "zeros" are smoothed out, making smooth frequency transition in the passband.

I hope this helps clears up a bit about how GMSK modulation works.

EDIT:

Oh yeah, about the geometric representation, it basically just turn clockwise when it is transmitting one symbol, and anti-clockwise when transmitting the other. BUT, it phase should be continuous, meaning it does not jump from one point in the IQ plot to another far distanced point at any time. AND, when it is about to turn clockwise from anti-clockwise (and vice versa), it does that smoothly, by first slowing down, stops, and slowly ramps up the speed of the reverse turn (to a maximum value, unless the bit changes again so that it have to slow down and start reversing again).