In this paper we investigate the deployment of LTE indoor femtocells on the frequency band currently used by GSM cellular network. We consider a scenario where the LTE femtocells share GSM uplink spectrum provided that the legacy GSM users are not harmed by interference from the femtocells. Difference in the characteristics of GSM and LTE technologies is exploited to enable the spectrum sharing. The performance of both systems is analyzed mathematically in terms of average SINR. Simulation is employed to support the analysis. Numerical results show that the availability of the spectrum sharing depends on the number of femtocells in each GSM cell and the locations of femtocells.