We study the secrecy capacity of Gaussian wiretap channel aided by an external jammer/helper. Both the transmitter and the intended receiver are equipped with a single antenna, while the eavesdropper and the jammer are equipped with $M$ and $N$ antennas, respectively. Generally, an analytical form of the secrecy capacity in this scenario is difficult to obtain. Instead, we consider a null-space jamming scheme which totally nulls out the jamming signal at the legitimate receiver, and derive lower and upper bounds on its maximal achievable secrecy rate ${R_N}$. The relationship between the average secrecy capacity ${\bar C_N}$ of Gaussian wiretap channel and the average secrecy rate ${\bar R_N}$ achieved by the null-space jamming scheme is investigated, and we prove that ${\bar R_N} \leq {\bar C_N} \leq {\bar R_{N + 1}}$. Based on this inequality and the derived lower and upper bounds on ${R_N}$, the upper and lower bounds on the average secrecy capacity of Gaussian wiretap channel aided by an external jammer can be obtained, where our result shows that when $N > M$, the average secrecy capacity increases linearly with the total transmit power; while when $N \leq M - 1$, there exists a performance ceiling on it.