On Topic, I have an ASUS PC desktop we purchased two months ago, which has a devilish combination of UEFI, MBR (even though 2TB) and MB/MOBO (whichever school you went to) constraints which have so far defied my efforts to install Linux, so I empathise. I'll crack it, sooner of later - it has Windows 7 on it, which I don't mind. Just got to fire it up this weekend to do the updates, but may take a couple of months before I tell it who's boss.
If it came with Windows 7, Secure Boot is disabled by default, as Windows 7 cannot run on a computer with Secure Boot enabled. You can verify that anywhere with anyone on the forum or off. Microsoft refused to issue Secure Boot keys for Windows 7, though they did to many Linux variants, which ticked off Linus Torvalds off in a very bad way (as an Advisor, I cannot repeat the terms, other than a type of contest they were participating in) he used towards the early distro leaders who chose to adopt this standard). This is because they want to make Windows 7 'the next XP', and has a bull's eye on it for extinction, but Windows 7 users won't lay down as meekly as XP users did. Linus didn't like it because the distro leaders were doing exactly what Microsoft wanted, pay for the keys, or you're not in. He had a better plan in litigation (better for all Linux users), but the spineless leaders of a couple of key distos took the bait, so didn't get a chance to execute it.
So you were done a favor & that's why there's no option to disable Secure Boot, because it hasn't been activated. Now if you were to install Windows 8.1 or 10 (not advisable), then it's possible you may be able to activate Secure Boot, or maybe ASUS left it out as an option, doing you a favor in the process. Or you may have to remove the keys from your distros to make them run.
One can have UEFI & GPT w/out Secure Boot, most computers that were manufactured in 2012 & before Windows 8 had both. Actually there's a performance benefit in GPT, not a large one, yet any performance boost should be considered a plus. And I'll repeat for the 100th or more time, GPT was here & in use 10 years ago, with Windows Server 2003 x64 & XP Pro x64, the catch was that one had to have all 64 bit OS's to run in GPT mode, though some says that they can't install a 64 bit OS on a HDD initiated as GPT on some older computers. I only tried it on two & was successful, but both were 2010/2011 model notebooks & neither has UEFI.
It could have been on the older ones (2005-09), these has to be business computers to install 64 bit OS's using GPT. Yet Linux systems could use GPT partitioned drives long before Windows could on most any 64 bit MB. Go to 'Operating Systems Support' & it's crystal clear that GPT was usable many years ago (Ubuntu 8.04 was some time back).