Unslice a string

Python 2, 49 bytes

lambda l:`map(max,*[' '*n+s for n,s in l])`[2::5]

First, lines up the strings by padding their offsets with spaces (shown as underscores for clarity)

[[2, "CG"], [0, "PP"], [1, "PC"]] 

__CG
PP
_PC

Then, uses map to zip and take the maximum of each column, which ignores the smaller values of spaces (the smallest printable character) and Nones where some strings were too short.

__CG
PP
_PC

PPCG

Finally, ''.join to a string using the [2::5] trick.

JavaScript (ES6), 61 bytes

a=>a.map(([o,s])=>[...s].map(c=>r[o++]=c),r=[])&&r.join``

Edit: Saved 4 bytes thanks to @edc65.

Perl, 25

Added +2 for -lp

Get the input from STDIN, e.g.

perl -lp slices.pl
2 CG
0 PP
1 PC

(Close with ^D or ^Z or whatever closes STDIN on your system)

slices.pl:

/ /;$r|=v0 x$`.$'}{*_=r