Epson does not claim their printer will reproduce the same color range as a RGB monitor which covers AdobeRGB color range. Epson claims that with certain paper and ink their printer can reproduce 99% of the color range used in this PANTONE color book:
The words in Epson's datasheet are carefully selected. There's actually zero information of how that 99% is calculated in math. Epson obviously hopes that people imagine under their scalps that the advertised printer prints nearly anything. But there's nothing which gives to the buyers a legal possibility to get their money back when they see, that in common indoor light conditions the printed colors do not look as vibrant as what's seen on bright large gamut computer screens.
I do not claim the printer is bad, probably it reproduces colors far better than common low cost home and office printers which use only CMYK inks. But only seeing the print results gives some info is it good enough for your purposes. Epson does not give such information in any exact guess-free format. I'm afraid that some hues are printed nicely and some others not at all so finely. But they surely have a math formula carefully prepared to show that there's no lies behind the claimed 99%.
As said by others, you can get installable AdobeRGB color profile for free. Today even GIMP can use it to show an acceptable estimate of the image shown in other AdobeRGB capable system. That's, of course, true only if YOUR monitor can display full AdobeRGB range and your system is properly calibrated to convert the RGB-numbers to visible light.
You simply open your image in GIMP and convert it to 16 bit AdobeRGB and save it as tiff to get what's wanted by the printshop.
But to see beforehand on your screen what's the output needs the color profile of the printer. You can well give an AdobeRGB image to be printed, but the colors are clipped or more likely flattened proportionally to what's physically possible.
That printer color profile should be used to preview the image in GIMP, Photoshop, etc... graphic program. Photoshop has offered this possibility longer than I can remember for common CMYK print processes and that's why people pay the premium for it. GIMP has recently also got that capability for previewing, but not for creating a CMYK file. Fortunately Epson's printers expect RGB files.
But how to get the proper color profile to be installed to your system for previewing? I do not know. Epson's support may answer or not. My best guess is that it comes with the printer driver software. Or it's not at all available. Start by getting the full user's manual for the right printer.
The printshop may have it. Or not. Ask if they can tell how you will get it, too. If not, they maybe can send to you a predicted AdobeRGB or sRGB image of the printed result. That's called sometimes soft proof.
