Question: 
Answer:
To estimate SAM accurately you need to practice time and motion study. Later, learn about motion codes and their respective time.
Apart from time study, is there any other way to estimate the SAM of a garment? I am struggling to estimate the garment SAM accurately as I don't have knowledge of using software like Pro-SMV and GSD.
Answer:
Answer to the first question - Yes, there is an alternate way to estimate the SAM of a garment other than the Time Study method. The option is using synthetic data. I mean use PMTS software.
Based on your second question, I see you already know about the alternative ways of establishing SAM. It is a PMTS system that can be used to estimate operation SAM. Like GSD Cost and Pro-SMV.
So, your main question is how to estimate garment SAM without doing a time study and without using any PMTS systems. You are looking for a third option.
I would say, there is no third option, but you can use a combination of both. Depending on the company's need you need to choose the one.
As you are working as an industrial engineer and working as an employee in a company, I would suggest learning time and motion study and improving your skills on time study. You can use the latest time study tools.
Regarding GSD Cost or other PMTS systems, it is the employer's responsibility to buy the system if they need accurate standard time for their products. Once the company buys the system, they will provide you the necessary training in the PMTS system. If your company is already using some kind of system, learn the system from your colleague.
Time study is one of the common methods used for estimating the standard time (SAM) of sewing operations and other activities. You can not ignore Time Study. Though it is a time-consuming process, the study of an operator's work through Time Study will give you a better idea than just SAM estimating. You can see their working pace, and skill in handling material and controlling the swing machines.
Other than time study, you need to learn motion analysis of hand and body movement while operators perform an activity.
To estimate SAM accurately you need to practice time and motion study. Later, learn about motion codes and their respective time.
If you think you don't have motion codes and their TMUs, you can use SPD (Sewing performance data) codes and their TMU value for Standard time estimation.
The GSD code and TMU value respective to the code are available online. I found someone uploaded a useful document on this. You can refer to that document for learning.