I'm using GMSK modulation to send $300 \ \rm bits$ and assuming $1 \ \rm bit/symbol$. $BT$ product is $0.5$. Sampling frequency is $100\ \rm kHz$ and bandwidth of the signal is $28 \ \rm kHz$.
The $BT$ product is linked to the single-side $-3\rm dB$ down bandwidth and the bit rate $R_b$ with the relation below: $$BT = \frac{f_{-\rm 3 dB}}{R_b}$$ Assuming you have a single-side $-3\rm dB$ bandwidth, you can extract your bit rate as: $$R_b = \frac{f_{-\rm 3 dB}}{BT}$$ Also see the answer to this question. For more on GMSK see this paper $^1$ and this paper $^2$.
You have one bit per symbol, meaning for every bit you have one symbol. And their rate should be in that proportion as well. Or see it this way, \begin{align} \text{For} \ \ &n \ \ \text{bits you have} \ \ n \ \ \text{symbols}\\ \implies \text{for} \ \ &\Delta n \ \ \text{bits you have} \ \ \Delta n \ \ \text{symbols}\\ \implies \text{for} \ \ &\frac{\Delta n \ \rm bits }{\Delta t} \ \ \text{you have} \ \ \frac{\Delta n \ \rm symbols}{\Delta t} \end{align}
See the sampling rate as the rate at which you are having your samples, and this value is in $\rm Hz$, or basically $$ \frac{\Delta n \ \rm Samples}{\Delta t} $$ The $\Delta t$s above are normally given as $1 \ \rm second$. The sampling rate is the number of samples per second, also known as sampling frequency (in $\rm Hz$).
Generally speaking, your bit rate is your data rate. The answer to this question details more on this.
You can use the maths and plug in your numbers.
$[1]$: A. Linz and A. Hendrickson, "Efficient implementation of an I-Q GMSK modulator," in IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, vol. 43, no. 1, pp. 14-23, Jan 1996.
$[2]$: K. Murota and K. Hirade, "GMSK Modulation for Digital Mobile Radio Telephony," in IEEE Transactions on Communications, vol. 29, no. 7, pp. 1044-1050, Jul 1981.
