The amount in CGAS is available for construction of your residential house. If the amount utilised is less than the balance available in CGAS, then the shortfall will be assessed as LTCG in the year 3 from the date of the transfer of your orginal asset. Thus if the asset was sold on, say, 20th May 2021 and the proceeds were invested in CGAS, the construction of the new house should have been completed within 3 years from the date of sale, i.e., by 20th May 2024. Now the following situtations may be arising in the usage of funds:
1) The entire amount was used in the construction of the house but the amount withdrawn from the CGAS was less than the amount used for the construction, meaning that the funds were used from own sources without using the funds from CGAS. In such case, you can apply to the Income Tax Department for withdrawl of the funds from CGAS.
2) The entire amount used for the construction is less than the amount claimed u/s 54F. In such case the shortfall, will be taxed in the year financial year, 2024-25, as the construction should have been completed by 20th May 2024. Now if the shortfall is, say, Rs 10 L and the shortfall in the utilisation of the balance in CGAS, is, say, Rs. 2 only, then you'll be paying LTCG on Rs. 10 L and the amount will be allowed to be withdrawn after the payment of taxes.
Trust this clarifies your query.