If I buy an unlocked GSM iPhone will I have to get a data plan?

My dad doesn't want to pay for a data plan but I have enough money to buy a gsm unlocked new iPhone 4s. I want an iPhone but he won't pay the data plan. So I thought if I could buy it unlocked and get a phone text and call plan that would do me fine because everywhere I go has Wi-Fi home, school, even mcdonalds and I wouldn't need it. My dad bought me 3g/4g hotspot for trips. So if I buy it unlocked will I have to get a data plan?

Nope. The carrier will detect the phone on the network & apply the data plan regardless.

If you opt for prepaid, then no.

For example, AT&T GoPhone Prepaid: http://www.att.com/...plans.html
- $25/month for unlimited text messaging, 250 minutes for calls
- $40/month for unlimited text messaging, 500 minutes for calls, 200MB of data.

T-Mobile Prepaid: http://prepaid-phones.t-mobile.com/prepaid-plans
-For example, $30/month for unlimited text messaging and data (first 5GB at 4G speeds), and 100 minutes for calls.

Hope this helps!

No, since you can connect to any internet then no need to get a data plan.
iPhone Unlock only means you can use it to any network that you like.