It's a bit laborious, but you can do it. Here's how:
1) Enable IMAP in your Gmail settings if you haven't already:
http://mail.google.com/support/bin/answer.py?answer=77695
2) On your iPhone, tap Settings.
3) Tap "Mail, Contacts, Calendars".
4) Tap "Add Account..."
5) Tap Other. (Careful, do not tap Gmail.)
6) Fill out the first screen as follows:
Name: your name
Address: your personal domain email address, i.e., the one you want to appear in the "reply to" field. Do not use your gmail address here.
Password: your gmail password
Description: a useful reminder of what this account is.
7) It will say "looking up account information" and then display an IMAP configuration screen.
8) Skip down to the Incoming Mail Server section and fill it out as follows:
Host name: imap.gmail.com
User name: your full gmail address including "@gmail.com"
Password: your gmail password
9) Under Outgoing Mail Server, fill it out as follows:
Host name: smtp.gmail.com
User name: your full gmail address including "@gmail.com"
Password: your gmail password
10) Press Save.
If you followed the instructions correctly, the account will be created. If you get an error, make sure that all the fields are filled in exactly as above.
Good luck!