What does 10 mean in libgmp10 library?

You’re referring to the libtool version, which is usually unrelated to the library’s release version.

A quick look at the sources would show you that the versions are set in Makefile.am, with a comment-block preceding the values giving a summary of the history.

Here’s a chunk where you’ll see the numbers of the Debian package (version info for libmp is in the last column):

#       release   libgmp  libgmpxx libmp 
...
#        5.0.1   10:1:0    6:1:2   4:21:1 
#        5.0.2   10:2:0    6:2:2   4:22:1 
#        5.0.3   10:3:0    6:3:2   4:23:1 
#        5.0.4   10:4:0    6:4:2   4:24:1 
#        5.0.5   10:5:0    6:5:2   4:25:1 
#        5.1.0   11:0:1    7:0:3     - 

and (current)

#        6.1.2   13:2:3    9:2:5     - 

and

# We interpret "implementation changed" in item "1." above as meaning any 
# release, ie. the REVISION is incremented every time (if nothing else). 
# Even if we thought the code generated will be identical on all systems, 
# it's still good to get the shared library filename (like 
# libgmpxx.so.3.0.4) incrementing, to make it clear which GMP it's from. 

LIBGMP_LT_CURRENT    = 13
LIBGMP_LT_REVISION   = 2
LIBGMP_LT_AGE        = 3

LIBGMPXX_LT_CURRENT  = 9
LIBGMPXX_LT_REVISION = 2
LIBGMPXX_LT_AGE      = 5