The power of your average Gem is a bit hit and miss...but in general, 4-5fpe is a good power to get from a .177, and up to 7-9fpe from one of the big .25's...would guess that .22 will be somewhere in between but don't know for sure, haven't got one yet!
Rifled Gems (like .22 cal Gems) are quite rare...but there are one or two about, however they are nearly always smooth bore...they can still give impressive groups at bell target ranges and beyond...I can shoot under 20mm groups at 6 yards off hand, in my garage...and, shooting off the bench, in perfect still conditions, as witnessed by my fellow club members at South Dorset Field Target Club...I've been known to hit a 6" steel fire alarm bell at 52 metres...all with a iron sighted Jean Marck No2 in .177 smooth bore and running sweetly at about 4fpe!
just realised that last bit sounds like an old PT Barnum advert for Gems
If your mate wants a good shooter then I can well recommend the JM Gem pattern, preferably in .177 if he can find one for sale that is!...
Gems patterns always wanted, especially Jean Marck T-Bar Gems