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The following information was written and provided by Nick Cernis, from his website http://modernerd.com/. You can read the full article from Nick here.
I’ve just taken the salient points and added them below in the interest of sharing the information as widely as possible.

1. Open the Settings app: 
2. Tap ‘Mail, Contacts, Calendars’: 
3. Tap ‘Add Account…’: 
4. Tap ‘Other’ (NOT Gmail or Google Mail): 
5. Tap ‘Add Mail Account’: 
6. Enter your full name in the Name field (this is what people will see in the “From” field when you send email), your Gmail address in the Address field, your password in the Password field, and the word ‘Gmail’ in the Description. Then tap Save. 
7. You should be presented with the ‘Enter Your Account Information’ screen.
8. Scroll down to ‘Incoming Mail Server’, and use ‘imap.gmail.com’ for the Host Name, your full Gmail address for the User Name, and your Gmail password for the Password. 
9. Under ‘Outgoing Mail Server’, use ‘smtp.gmail.com’ for the Host Name, your full Gmail address for the User Name, and your Gmail password for the Password.
10. Tap the ‘Address’ field once to place the cursor at the end of your Gmail address: 
11. Now, start typing one of your secondary email addresses that you’d like to be able to send email from, each separated by a comma. To type a comma on the iPad when you’re in an email field like this, first tap the ‘.?123’ key followed by the ‘#+=’ key. The comma will appear in the top right of the keyboard, to the left of the backspace key.
12. Repeat iPad steps 10 and 11 for as many email addresses as you like. You should end up with a list of email addresses, separated by commas. The field should end with a ‘.com’ or ‘.co.uk’ or some other domain, not with a comma.
13. Tap ‘Save’.
14. Tap your iPad’s Home button to get back to your Springboard.
15. To check it’s worked, open Mail, browse to your Gmail account, and create a new message.
16. Tap the “Cc/Bcc, From:” field to open it up, then tap the “From:” field. You should see a picker overlay containing all of the email addresses you just entered. If not, don’t panic — just power down your iPad by pressing and holding the power button, and it should be there when you next power up and use Mail.

Final notes

Note that recipients will still be able to see your original Gmail address when you send from your secondary addresses by looking at their email header information (just like Gmail in your web browser). This shouldn’t be a concern for most, but if it is, you need to set up completely separate mailboxes instead of forwarding all of your addresses to one account.

Also note that, in step 13 of the iPhone instructions above, I suggest using the Search field to copy and paste a comma. You can actually use any field that brings up a full keyboard, including those in the Settings app itself. I just felt it was slightly clearer this way.

Finally, I prefer not to answer email about Gmail settings and configuration, if only because it’s nice to pretend that I’ve got more important things to do. If you need help with this stuff, try the Apple discussion board or Gmail support forums. Thanks.