The 4L60E has an internal thermistor to provide a temp to the TCM to control shift points. The temp is available through the PCM but not as a signal to a sep. gauge.
I have seen people add a temp sensor into the test port on the left side of the transmission case. The temp reading there would be approx 10 deg higher than the fluid in the pan. The port is close to the shift linkage so care must be used to make sure the sensor and wires do not contact the linkage.
If you want do this, I would run it by Greg Ducato on this site to get the correct sensor, gauge and install instructions.
The other option is to have a bung welded into your pan for a sensor.
Don't be tempted to tee into the line to the cooler, that risks adding several potential leak points that could lead to a transmission failure.
Greg is right..... the 4L60 has a sensor buried in the valve body of the transmission already.
The simplest method of measuring trans temp (other than installing the elusive 03-04 console gauges) is to get the Bluetooth plug-in for the OBD-2 connection above the driver’s left knee. This will make ALL digital measurements available to you. Connecting up your cell phone with one of several available apps will give you absolutely dead accurate digital display.
You can also install a cast aluminum tranny pan with a built-in boss for a temp sensor. Drill and tap the boss to match whatever temp sensor you choose. The gauge you install is up to you.... lots of old school possibilities on both components.