How to have a GUI in the same place for everyone?

Every GUI has a property named Position and Size. Position and Size are divided into two categories, the X and Y axis. The axes are divided into two more categories, the Scale and Offset. Scale (the first number) resizes the GUI based on the percentage of the screen from a scale of 0 to 1, and Offset (the second number) uses pixels. The Size and Position are organized as (X-Scale, X-Offset, Y-Scale, Y-Offset).

Position

Let's say we want our GUI to be dead center in the middle. We would edit the Scale so that it is 0.5 (50% of the screen). This gives us a position of (0.5, 0, 0.5, 0). (NOTE: When centering GUIs, always remember that the top-left corner of the GUI is used as the reference point.)

Size

Let's say we want out GUI to span across the screen horizontally but take up half the screen vertically. We would edit the scale so that the X-Scale is 1 (100% of the screen) and the Y-Scale is 0.5 (50%).

If this was unclear, just ask me a question and I'll edit my response. If you thought my answer was helpful, you can upvote and accept it. I hope it helped!

Example

Some people use the Scale for size and position while others use Offset. It's a matter of preference. Personally, I use Scale all the time because it adapts to all screens. But let's assume that you want to use Offset for your GUI. Let's use the second GUI for demonstration. First, set the GUI's position to the bottom-right corner. It's not as simple as changing the Scale for Position to 1 because the GUI measures from the top-left corner and your GUI will end up outside the screen! So we should use the Offset property to push it onto the screen. The GUI's size is {0, 250},{0, 50}, so the GUI's Position should be {1, -250, 1, -50}. You said you wanted to push it 10 pixels off the screen though. Just add (or subtract!) 10 to the offset to achieve this. The GUI's Position is now {1, -260, 1, -60}! Note that if you were using Scale to resize GUIs, you wouldn't use Offset. I hope this way easy to understand! It's kind of hard to explain in words.

EDIT

In response to your edit, no, you cannot swap the Offset and Scale and expect satisfactory results. Note that when using Scale, any numbers over 1 will not show up on the screen, because 1 = 100% of the screen. Assuming that you have knowledge of basic number to percentage conversions, setting the Scale to 120 will place the GUI at 12000% of the screen. The problem with this is self-explanatory. Same with 675, which would place the GUI at 67500% of the screen. The same applies to Size. As a reminder, 0.2 = 20%, 0.4, 40%, 0.75 = 75%, etc.