Can you use a Cricket SIMS card in a T-Mobile iPhone?

In other words, are tmobile iphones factory unlocked. Or gsm?

I don't believe that T-Mobile phones are unlocked. You have to contact T-Mobile and have them unlock it.

There are some requirements that you have to meet in order to get them to unlock it, though.

Device eligibility is determined as follows:
The device must be a T-Mobile device.
The device must not be reported as lost, stolen or blocked to T-Mobile.
The account associated with the device must not be cancelled and is in good standing.
You have requested no more than 2 mobile device unlock codes per line of service in the last 12 months.
The device must satisfy all the Postpaid or Pay in Advance (Prepaid) Unlocking terms outlined below.
T-Mobile may request proof of purchase or additional information in its discretion and certain other exceptions may apply.

Tmobile phones are GSM, but they're locked. Get your cricket phone unlocked and put a tmobile sims card in it and activate their service.

I buy unlocked phones online and buy a tmobile SIM card and activate my service that way.

I JUST REREAD YOUR QUESTION: the only way to use a cricket sim in a tmobile phone is to get the tmobile phone unlocked. If you don't owe anything on the phone and have no debts to tmobile then it has to unlock it upon request. It's the law.

T-Mobile phones are not unlocked, you've to unlock them for using on other carriers like Cricket. Both the carriers, T-Mobile and Cricket, are GSM carriers. If you need help with getting your phone factory unlocked, mail me for more info.