What is the best possible way to persuade the GM to make a change in the entire company's structure with minimum down time and costs?

Well, first of all, you need to be certain that the problems are too big to be fixed without a re-organisation to the company. Last thing you want to do is fix what's not broken, and be responsible for negative consequences.

If you are sure about the problem(s), you need to tell the responsible person about what it is, how big it is, and what's the impact to the overall business, then you need to have the to-be model, and provide the pros and cons to it (SWOT analysis is a good way to present). Then address the 'how' (how to transform from current model to future model).

Probably you won't be the person responsible for the change, or to suggest such changes to management. If there is someone who has the task of business/strategic planning, you need to first suggest to him/her before going to senior management, if the GM is the person who does the change, then you can present it to him directly, after your managers’ approval.

The best way, IMHO is to propose a plan/suggestion/idea and provide at the same time a solution or roadmap for that. Keep in mind that 'change' is a word that everyone is trying his/her best to avoid for the first impression so you need to avoid using that word as much as possible and be positive as much as you can. I have managed to do some changes wherever I worked but these were not a major changes for the entire company. You will need a very solid plan for that :)

Whilst your proposal is of substance and a great idea forward, the MG and everyone else has other problems to attend such as budget, meetings, deadlines and crisis, you idea is an additional item on already overcrowded list.

With the above in mind try to make it easy for the MG and your colleagues to accept your proposal and this is how you do it;

Demonstrate your idea will make it easier for them to achieve their goals showing the opposite scenario: opposing your idea will take up huge amounts of their time and will result in lost opportunities (play the risk factor) Make it difficult for them to say no.  

with the best plan that you have coupled with your expertise in communication, schedule a meeting with the GM and discuss and present to him / her the benefit-analysis that you have in making changes with the entire structure of the organization. the analysis should always have to favor the organization in terms return of investments, minimal changes in the workforce, and the implementation of efficient office-wide application system that will generate income and revenue, speed up works and reports, which offers a lot of advantages to the organization as compared to the old structure .

To all great professionals here,

Thank you all for sharing your great ideas with me ,each and every one of you has added something new and beneficial to my prospective. .. Many thanks

First its extremely risky affair, you need to really be ready with any type of reaction, you obviously know the reason why?

In case you still want to take a shot then first you need to prepare a breif report on where do you stand currently (sales, growth, issues, etc.), then you will have to point the gaps in all the areas where you need to improve, including manpower. Final step would be proposal which should cover all gaps, positive changes, growth you will expect with this change. If you can explain well, then you have good chance of approvals. 

first: we need to define the problem then identify how to solve the problem by define another option then we need to make the comparison between them which is the best then make a decision and monitor the results

Answer is so simple make a brief structure and Analysis and correct the mistakes and put it on the table for response.