Try not to weld them onto the sliding gate frame. It will be a nightmare to replace them later.
Measure the sliding gate wheel cradle allowing enough room to attach the sliding gate wheel to the steel frame, I use a cutting wheel but first drill 4 holes, one in each corner so the square hole is clean in each corner using 8-10mm drill.
When you start to screw in your tek screws make sure the sliding gate wheel is in line with the edge of the steel, the last thing we want is the wheel to be not in alignment. Don't weld the wheel if you can help it this only makes it harder to replace them later on.