The most common and most likely answer to this crossword clue is the 10 letter word BIRTHSTONE
I've seen this crossword clue in The Daily Telegraph
B
I
R
T
H
S
T
O
N
E
More clues leading to the result BIRTHSTONE
Buddy explains!
The straight part
We know that "Gem of a baby" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gem of a baby = birthstone.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BIRTHSTONE mean?
BIRTHSTONE noun- Any of twelve gemstones traditionally associated with the month of one's birth.
- Any of a similar list of less costly substitutes.