About difficulty, if you are trying to make the bullets faster, you will need a few variables, and maybe another object. I would have another object that would be a controller object. It would store variables for things like the current difficulty, and could also store the current bullet speed if there is only one type. Then, in the create event of the bullet, you would access the variables in the other object and use that speed instead of just using the default speed.
Your controller object would then track whatever you need it to in order to increase difficulty. If it is a matter of time, then you could have an array(a concept you should learn soon anyway if you don't know it) to hold a group of bullet speeds based on difficulty. If difficulty is level 1, then access the first value in the array, which would be the lowest speed. Then if you increase the difficulty number, it accesses the numbers further in the array, which you should have as larger numbers for the bullet speed to be faster. Now, since you have decided on time as the factor, say after 60 seconds, you use an alarm event(set to 60 times the room speed amount of steps), and increase the difficulty at that point. You also have to have a check somewhere, say if you have 5 difficulty levels, the speed never goes higher than that.
Another option is to use some formula to calculate things. It takes a bit of math to figure it out, though you can do things simple too. For example, you could say the bullet speed is simply a formula based on how long the game has gone, with a max of some number. Say you start with a speed of 2, and want a max of 10, increasing 1 per minute, then somewhere you store in your controller a counter for how many steps have gone, and each time it gets to a minute worth(depending on your room speed), you add one to another variable for minutes. When you create a bullet, you set it with the speed of 2+(the minute variable). Somewhere, either the minute variable, or the bullet speed setting, you have to designate the max, unless you expect the player to never get to the point where the speed would be too high for gameplay purposes.
As far as contacting me for help, I don't mind. I check the forums often during the evenings(US Central Time, I'm in North Texas), but I work all day and usually get home around 6-7PM, so if I don't respond right away, that is why.
And also, I understand completely you wanting to use GMS over Unity. I used it for years, not so much because I couldn't use Unity, but because of how quick and easy it is. Eventually, I got to wanting to work on things that GMS is simply not good at and needed to make the switch, but if I want to make a project that fits GMS I will be glad to use it, the best tool for the purpose.