It's based on ESR r9b (not yet published, I didn't have time to make proper testing, so most likely it will be released tommorow).
Launch CDVD - it will launch PS2DVDV/PS2DVD in drive directly (also original PS2CDs) - just as in "dvd direct/direct off" versions of usual ESR.
Launch App - it will launch application in mcx:/BOOT/BOOT.ELF or mcx:/B?-DATA-SYSTEM/BOOT.ELF, where x - memory card slot, either 0 or 1; ? stands for regional code: E, A, I. Application will gain DVDV reading support (just as in "mcard" version of the usual ESR).
Launch OSDSYS - it will simply launch OSDSYS, so with FMCB installed, it will relaunch FMCB.
Disable ESR after first IOP reset - does exactly what it means. Just as in the "off" version of usual ESR.
Use Synchronous Mode - will use synchronous reads, some incompatible games need that. This option saves a lot of IOP memory. This will make the reads to be slower and FMVs sometimes to be choppy. Some games will hate this option and will refuse to work at all.
Spoof Media Type - always returns "PS2CD" media type to the EE side. It is just a direct followup of automatic media mode patching seen in earlier ESR revisions. With this option, most cd converted games will not need anything else (no patches, etc.) - this doesn't apply to games that are incompatible due to bad conversion (static LBA tables or other protections).
To play "Jak and Daxter: Precursor's Legacy", which is incompatible with all previous ESR revisions choose:
"Disable ESR after first IOP reset" (to prevent this game from crashing randomly at cutscenes)
"Use synchronous mode" (to make it work at all)
Now just use "Launch CDVD" and it will work.
Changelog for the yet unreleased ESR r9b which this GUI is based on:
Note, that this GUI is nothing more than a test. It's unfinished and most likely has many bugs... but should work fine in most of the cases.