Connect a 5mm led to a GPIO without transistor?
Under some assumptions, this will work well:

You don't care about a precise LED current and brightness.
This is a one-off project.

You need to look at this graph from the datasheet:

There is a corresponding graph for a VCC of 3 V as well.
Naïvely, you will take a quick look, see that it can source 20 mA, and leave it there. Note however that the output voltage drops to a volt or less. What that means is that your LED won't ever draw 20 mA from this port. As long as the LED will try to draw more current, the voltage will drop, until there is a balance between LED current and LED voltage.
All this means that it's almost impossible to select a good &quot;dropper&quot; resistor. Especially considering you plan to run it from 2 V to 3 V. At 2 volts, you really don't have any headroom.
If I were you, I would take advantage of the table on page 18 - Outputs - Ports Px.




PARAMETER
TEST CONDITIONS
VCC
TYP
UNIT




VOH (High-level output voltage)
I(OHmax) = -6 mA
3 V
VCC - 0.3
V


VOL (Low-level output voltage)
I(OLmax) = 6 mA
3 V
VSS + 0.3
V




Assume a LED current of 6 mA, assume that the voltage on the pin will drop 0.3 volts, then calculate a resistor value from there.