The Gems aren't worth much as incomplete guns I'm afraid...may be £20-30? may be not ...that's if they are _not_ totally knackered, pitted, rusty relics with sloppy breeches.
The piston and cylinder of a typical Gem pattern is usually machined to fit (piston and cylinder are matched together) so swapping pistons around isn't usually possible without a lot of work, mucking about to make it fit and/or making a new piston head (I know, I've done it a few times). A lot of the other spares are quite specific to each model of Gem too, again requiring work to modify for use in another Gem (even the same model! )...It's not impossible, but to a lot of folk it's just not worth all the hassle to end up with a non-original 'bitsa' gun that is still only worth £30-50 max!
Having said that, if they are T-bar Gems with fully octagonal barrels and swiss target sights fitted? ...then they are possibly Jean Marck guns and are then possibly worth a bit more to nuts like me!...and that is not an offer to buy, just an observation!
Edit: ...I've just noticed the link to pics!...both Gems appear to be the pretty common Langenham/Diana 3L type Gems made just before WW1 (the flat section trigger guard is a dead give away)...Although one is a dollshead and one is a T-bar, they were both sold as the 'Diana 3L' ... lending weight to the theory that these are re-badged Langenhan Gem patterns (these are usually given a number designation in other retailers catalogues of the time). The good news is that you may get lucky and be able to swap the trigger guard straight over from the dollshead to use on the T-bar Gem...although chances are that the sear and maybe even the trigger will require some fettling to make it work...you may even get lucky and be able to swap the piston from the T-bar into the dollshead...as these types sometimes do have a piston washer fitted so it is easier to make one fit the other...
They look savable, so I'll stick by my original estimate of £20-30 for the pair, they are not for me though.
Goodluck with the sale