That's funny. You have all these random operators that you're constantly adding. I think you would have more people choosing Continental for an operator than "UAB Handelshus".
Those operators are not "random", they are always added following users suggestions of ACTUAL operators that ARE currently serving real life airports today so, fact they appear "random" to you, doesn't mean they are not real and not important to other users, because GSX is clearly a global product, so all countries are important, more than historic airlines.
Is there any way to create custom content?
Of course there is, it's called "GSX add-on packages", to know how it works, you should join the GSX Community server on Discord, which has a channel dedicated to creators, with a PaintKit to create new liveries that can be installed as separate GSX add-on packages.