Display Gmail, Yahoo or any other mail list in codeigniter Application.

I gave you a point to the right direction. Try to do it yourself and if you get any doubts, post it here on the forum.

I have our admin panel on our website where all the customer orders come and we correspond with our existing customers through the Gmail

I need out admin panel to be merged with gmail, 

this is because when we get email in Gmail and we need to respond to that customer , to find their details, I need to copy and paste their email address on Admin and then I will know about their order and then have to go back to Gmail, if there is a way to merge gmail and admin panel, we need to get some help from codeigniter.