New build with recycled parts on a ridiculous budget 120cm*60cm work area

[Zeeflyboy]

I have no idea how to make sure the gantry is properly square though in the first place, pretty certain a set square isn't going to cut it lol.

Machinist's square is one option, but they are expensive. I got a 200mmx130mm one (square to 0.005mm/150mm or better) from starret for about 80 quid with a 20% off code.

You can make a less perfect square work by flipping it and averaging the results, but once you get into the lower ranges of measurement just the flatness of the edge can become problematic and that's harder to cancel out.

You can also drill a square with diagonals that you can measure with eg a digital calliper (100x100mm square will give an easy to measure distance that your typical digital calliper will reach). Insert some dowels and measure the corner to corner distance. If both corners are equally spaced diagonally then it's square, if one is longer than the other then it's forming a parallelogram and is leaning towards the top corner of the longer diagonal...

One problem you'll probably encounter, which I also have on my current machine, is that your extrusions aren't particularly flat or straight. You will have varying curves along the axis rather than a perfectly straight line which makes fine tuning the squareness extremely difficult if not impossible as it varies from one section of the bed to another... so you just have to do the best you can.