There is a fan control relay on that vehicle; which could cause this problem, if it were defective. Two different models were used; depending on the whether or not it has rear air conditioning. (AC Delco # D954A used without rear air; or AC Delco #158733 used with rear air.) The relays are inexpensive enough that it may be easier to just replace them; rather than trying to hook up jumpers. The fans should come on when the temperature gets hot; or when the A/C is on Max.
Since the coolant was drained when the intake gaskets were replaced; there might have been an air pocket left in the system. For that reason; it would be important to remove the radiator cap when the engine is cold; and make sure the radiator is filled all the way to the top with a 50% mixture of coolant and distilled water. The plastic reservoir bottle should also be filled to the normal level with the same mixture. If the level dropped during the first day or two, due to air pockets coming out; that may cause the temperature to go up. That's why it is important to check the level in BOTH containers. Once the coolant level stabilizes; the level can be just checked at the reservoir (unless the reservoir has become empty).
Hopefully, the thermostat was replaced when the gaskets were changed. If not; I would suggest having it replaced.
You ought to know that there was a massive class action lawsuit against GM because the Dex Cool coolant they required turned out to damage those intake gaskets. Your vehicle is eligible for reimbursement for some or all of the cost of this repair. See the website referenced below.
Source: http://www.dexcoolsettlement.com/
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