in these situations, I'm a fan of using multiple speakers for coverage, rather than bludgeoning the space with a single point-source speaker. Question #1 is: where is this single speaker located? Is placed in order to cover the listening area equally? If not, and some of the audience is close to the speaker with others far away, then you'll be cranking to accommodate the furthest reaches while melting the faces of those close by.
An alternative thought: 3 or 4 very modest-but-adequate powered speakers ($250 ea+/-) on tripods will do better in these types of environments than putting all eggs in one basket, spending well upwards of $1000 on a single, high powered speaker.
This is a matter of covering the area adequately with intelligibility more than anything else. Perhaps other scenarios would be better suited to the "loudest speaker I can get" train of thought, but perhaps IMHO not this one.
This is the answer. Ideally you need speakers with very tight pattern control over their entire bandwidth, so you can aim the sound on the audience like a beam coming out of a flashlight. The idea is for the audience to hear primarily direct sound coming from the speakers. Unfortunately real world speakers generally become less and less directional as the frequency decreases. The lowest frequencies are basically omnidirectional and shoot out in all directions.
Since a gym is large, and has a lot of flat surfaces and very little acoustic treatment, low frequencies tend to bounce around for a long time and create what is called a diffuse field. A diffuse field basically means the sound is moving in all directions randomly and the SPL is uniform throughout the room. This distance in which direct sound is equal to the reverberant sound is called the critical distance. In order to increase the critical distance you need to keep as much off the sound of the floor, ceiling, and walls as possible.
The tools you have are speaker aiming and possibly an HPF. The K12.2 has a built in tilt adapter, so get the speaker up high and then tilt it to aim down into the audience. Next run the HPF up as high as you can without gutting the sound source. Remember the speaker has better pattern control at higher frequencies, so the idea is to cut the lower frequencies that are unnecessary and shooting off at wider dispersion angles to bounce off reflective surfaces and feed the diffuse field. If you are using the mic for vocals I would start with an HPF up around 180hz and increase it until the lowest range of the vocals starts to thin out. Depending upon the vocalist, you may run the HPF up over 300hz, or if the vocalist is a baritone you may have to drop the frequency a bit. In my experience, it's extremely rare to need to run the HPF below 180hz though.
FYI, the dispersion of the K12.2 is listed as 75 degrees, so use this for course aiming of the cab by eye, and then use your ears to confirm the aiming. Obviously 75 degrees is not a wide enough dispersion pattern to cover gym full of people. Ideally, you would have a row of speakers run across the face of the intended audience. By running multiple speakers closer to the audience, you can actually turn the level down so less energy is fed into the diffuse field. The minimum distance from the audience and spacing between the speakers would be determined by the 75 degree dispersion pattern.
Of course I realize cost is a prohibitive factor, but I think your best best is to buy another K12.2
