01 Monte won't get up to temp

[supersteve1191]

Just replaced upper and lower intake gaskets and now the car won't get up to temp. The gauge barley moves when you turn the car on and doesn't go past the lowest mark which is 140. I was thinking a thermostat but before I replace that I wanted to see if anyone else has an input. I know those intake gaskets are common with the 3800s

[TheBigMortboski]

Thermostat is usually my first guess with that problem. Its easy and cheap, so its a good way to start.