The ReplayGain info isn't supposed to be displayed that way, if memory serves, so it's probably not something foobar2000 will one day do by default. And 100dB should at least be labelled Track Volume, not Track Gain.
The popup window displaying the results can't be changed, but I often found myself wanting to see them again after I'd closed it, so I created a tab containing the Playlist View element for displaying volume. Chances are you won't want to get that carried away, but you could create a custom playlist column for your current setup, and once the tags are updated, it might make it more accessible.
This should display the volume in the way you asked. I don't use it myself because it's mental, but I think it's just a matter of subtracting the TrackGain value from 89 and making it display nice.
ReplayGain Volume (calculated from track gain).
$if(%replaygain_track_gain%,$puts(TRACK_VOLUME,$sub(8900,$replace(%replaygain_track_gain%,.,)))$ifgreater($get(TRACK_VOLUME),9999,$substr($get(TRACK_VOLUME),1,3).$substr($get(TRACK_VOLUME),4,5),$substr($get(TRACK_VOLUME),1,2).$substr($get(TRACK_VOLUME),3,4)) dB)
The ReplayGain target volume of 89dB is a sound pressure level and rather meaningless to mere mortals. If you're interested, this is the syntax I use for displaying volume in a normal manner.
Volume (calculated from track gain).
$if(%replaygain_track_gain%,$pad_right($puts(TRACK_VOLUME,$sub(-1800,$replace(%replaygain_track_gain%,.,)))$ifequal($get(TRACK_VOLUME),0,0,$ifgreater($get(TRACK_VOLUME),0,$replace(+$substr($num($get(TRACK_VOLUME),4),1,2),+0,+),$replace($substr($num($get(TRACK_VOLUME),5),1,3),-0,-))).$substr($num($get(TRACK_VOLUME),5),4,5),6) dB)
ReplayGain's 89dB target volume is equivalent to -18dB or -18 LUFS in human-speak, or -18dB on an output meter. The TrackGain and AlbumGain values still work the same way, but you can think of them as being relative to -18dB, instead of relative to a volume that doesn't make sense.
Anyhow, to test the syntax for displaying volume as ReplayGain mumbo jumbo, I added a column to my setup, and attached a pic so you can see what I'm talking about. The "ReplayGain Volume" column is displaying it in ReplayGain-speak. The "Volume" column is displaying it as the rest of the world does.