Soon I am going to replace my 2 prong outlet with a 3 prong GFCI that will protect downstream devices. I am wondering to test the line and load if it is ok to use Push-In connectors Gauge 22-12. I just want to use them while testing. I appreciate any help.
I don't believe you're referring to the push-in terminals on the receptacle, but push in connectors like these:
In my experience, this type of push in connector is a safer bet than wire nuts for someone that isn't practiced at using wire nuts. I have not seen any real problems with these, and they are very easy to use.
However, I would not use them for testing. You have to pull and twist them to remove them, and it dings up the conductor a little bit.
To simply cap single wires, insulating the stripped bit at the end for safety, wire nuts are fine for anyone - while it takes a little skill to get a good splice with wire nuts, anyone can cap a single wire - as long as the wire nut isn't too big.
For testing when you may have to unmake and remake splices, lever nuts are best. They are easy to use and easy to remove.
I know quite a few electricians that use push on connectors. I don't Personaly like them because they can be hard to remove (pulling and twisting usually works) I have had two many back stabs on mostly outlets but some switches fail in the past and these were limited to 15 amp circuits #12 wire could be on a 20 amp circuit but I know they can be listed for use on a 20 amp circuit.
Push-in splices, like backstab connectors, are a major source of connection problems down the road, where the circuit just goes dead for no apparent reason.
They are also a major source of arc faults and the fires which follow from them. They the primary reason almost every circuit must now have a $40 Arc Fault breaker on it.
The point of any kind of jab-and-forget connector is speed. (It isn't surety, obviously). They cut the time of wiring a new-being-built home by like 20%. For repairs like yours, the time they save is like 1% of the overall job. Totally a false economy and not worth it.
Also, backstabs are single-use, so once you jab, that hole is finished and must not be used again (or it will fail quickly). The receptacle types are only good for #14 wire.
Not to be confused with "screw-to-clamp" connections where the wire fits loosely in a hole directly behind the screw, and you tighten the screw to clamp the wire. Those are fine if you set torque to spec with a torque screwdriver, can fit any wire they're rated for (#22 should be ok for testing) and are reusable.
Or the splice block equivalent with levers.
Otherwise, use wire nuts properly, or the side screws with J-hooks like normal.