[font=trebuchet ms]These two lines are what you need;[/font]

Output Power Rating: 115 watts RMS @ 5% THD 4 ohms load 120VAC
[font=trebuchet ms]There's usually a spec for the minimum load impedance, but in this case we can assume that 4 Ohms is it. The 120 VAC is the mains power, not your amp output.[/font]

Speaker Size and Rating:GX-212: 2 Crate Custom 12”, 8 ohms
[font=trebuchet ms]This shows that the standard cab has two 8 Ohm speakers wired in parallel to give a 4 Ohm net impedance.

So you can use any 4 Ohm cab that will handle 115 Watts rms. Because your amp has a transistor output stage, you can also use any 8 or 16 Ohm cab, but the amp will deliver less power to a higher impedance load. If you decide to run two cabs, they will be connected in parallel. You can use the formulas I posted earlier to calculate the net impedance of these cabs. Just be sure that the net impedance is at least 4 Ohms.[/font]