In this paper a G-Let based steganographic technique, termed as GASMT has been proposed for authentication of gray scale images. The cover image transformed into one to many G-Lets based on group theory. n number of G-Lets are embedded with secret message/image for authentication. Using a secret key and hash function watermarks/secret are embedded into few G-Let components of the cover image followed by adjustment/tuning of embedded G-Lets components to minimize the noise integration. At the receiving end embedded G-Lets are used for authentication. Experimental results are computed and compared with the existing authentication techniques like Li's method, SCDFT, Region-Based method based on Mean Square Error (MSE), Peak Signal to Noise Ratio (PSNR), Image Fidelity (IF), Universal Quality Image (UQI) and Structural Similarity Index Measurement (SSIM) which shows better performance in GASMT, in terms of low computational complexity and better fidelity.