How to email out using a different email address using gmail on my iPhone?

I have a gmail account for work but email from that account using my @companyname.com email address.
How can I send emails using @companyname.com from my iPhone rather than @gmail.com?
I will still need to use my gmail account, but will need to email out using the different address.

I use the exchange settings for my google account on my iPhone 5

You set it up manually on the iPhone using:

Settings > Mail/Contacts/Calendars > Add Account (at the top of the list).

You then need to manually input the settings, so you will need the password for your @companyname.com email address as well as the POP3 server settings.
You would be able to get all these settings from your @companyname.com internet server (perhaps your works I. T department can help?)