"geil" has two distinct meanings.

The first meaning is "randy" or "horny" or "sexy".

The second came later and resulted from the first just like the second meaning of sexy resulted from the first.

Because anything that is geil or sexy is good, it became possible to characterize something very good as geil or sexy.

For your context, some equivalent modern youth expressions if the situation has nothing to do with sex are:

FAB

FUNKY

BRILLIANT many Google hits

BRILL – 8 (short for BRILLIANT, my very up-to-date 14 year-old niece in Manchester)

TIGHT - 113 (used by many US kids 8-11 years, via a niece in Maryland who teaches them)

ALL THAT – couldn’t be properly searched, the same US kids)

AWESOME – 41

GROOVY – 14 – (Yeah, I know it's old but the niece in Manchester uses it!)

COOL – 187 – (old-fashioned but apparently still alive and kicking too)

WOW – 75 – (old-fashioned but apparently still alive and kicking too)

FAR OUT – couldn’t be properly searched

SUPER – 154 (old-fashioned)

DA BOMB – Zero hits – (the same US kids)

HTH

Dan