Use Q5 DNA polymerase from NEB according to the company's manual. its very robust https://www.neb.com/products/m0491-q5-high-fidelity-dna-polymerase#product-protocols

If this gene has moderate GC-content, there is a possibility to amplify it as a single fragment. However, it will require very long elongation time (e.g. several minutes). Special polymerase or polymerase coctail to avoid amplification errors also will be needed.

If GC-content is high, some chemicals like betain or DMSO may facilitate PCR, but they increase error rate of polymerases. DNA-binding proteins (SSBs) also may help. In this context, the simpliest way to obtain such long fragment may be to divided it into several shorter fragments and amplify them separately, following by hydrolysis with appropriate restrictases and ligation.

Hi Mangal

Thanks. I have already tried with Q5 Hotstart DNA polymerase. Is it different from Q5 DNA polymerase? Do you have any experience to amplify the long frangment using the Q5 DNA polymerase?

Both are same the only difference is the hot start feature which prevents non-specific amplifications and you can assemble your reactions at room temperature too. Initially I used the hot start Q5  and then shifted to normal Q5 as it was cheaper but requires more effort (keeping the samples on ice). I have recently amplified a 3.3 kb DNA fragment from a high GC template, successfully cloned and overexpressed it.