Hello, it is very easy to find a solution to your request.
I'll quickly explain the timeline of Motif workstations. As you well know Montage/MODX reads files from Motif XF / XS, which read files from Motif ES.
Motif ES reads files from Motif Classic, of which PowerGrand is a part.
That is, you do not need any samples.
Go to Motifator's site, there good people have converted completely free Motif Classic Factory Sounds to MOTIF XS X0A format, which your Montage/Modx will read. There you will find your sound, as well as many others. 🙂

Ignore that first reply - it is from a spambot.

What am I doing wrong?

You aren't doing anything wrong. That library is NOT compatible with Montage/Modx. See this thread for details.
https://www.yamahasynth.com/forum/difficulty-loading-a-x6a-library-inspiration-in-a-flash-onto-a-modx6

This sound pack claims it has the PowerGrand. Likely sampled from the output of a Motif XF. I'm surprised this wasn't done earlier by the community since Montage gets Sample Robot at no cost. The licensing/rights question is a little sticky - but still surprised.

https://vgsgmusic.com/motifgospel
also available (more packs combined including above -- fits in MODX+ and Montage only - not MODX):
https://vgsgmusic.com/fullflash

On the free side there's "Worship Power Grand" - which, in the youtube comments, someone mentions "Power Grand!!". There's a dropbox link that is currently working to get this for free. The same provider makes a lot of MODX/Montage packs available for various other pianos and EPs.

https://www.youtube.com/watch?v=d3zNmKRcMeo

... a google search found these. There may be more options - I just picked the first two. I searched for "powergrand montage" using Montage instead of MODX because Montage has been around longer and is highly compatible with MODX.

EDIT: Updated a typo

Thank you very much, guys!:)

http://www.motifator.com/index.php/forum/viewthread/473177

I found the post, there you will find the file ClassicMotifVoices.n3.X3V
I hope I was helpful