gdb 7.8 is currently not available in trusty repo. But you can install it from the source.

Open terminal and type following commands

wget http://ftp.gnu.org/gnu/gdb/gdb-7.8.tar.xz

tar -xf gdb-7.8.tar.xz     

cd gdb-7.8/     

./configure

make

sudo cp gdb/gdb /usr/local/bin/gdb

It will install gdb in /usr/local/bin/ directory. As /usr/local/bin/ is searched before /usr/bin/ whenever a command is executed, running gdb will execute gdb 7.8.

Once installed, you can check gdb version using

gdb --version

It should output

GNU gdb (GDB) 7.8

Copyright (C) 2014 Free Software Foundation, Inc.

License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software: you are free to change and redistribute it.

There is NO WARRANTY, to the extent permitted by law.  Type "show copying"

and "show warranty" for details.

This GDB was configured as "i686-pc-linux-gnu".

Type "show configuration" for configuration details.

For bug reporting instructions, please see:

<http://www.gnu.org/software/gdb/bugs/>.

Find the GDB manual and other documentation resources online at:

<http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".

Type "apropos word" to search for commands related to "word".

If you want to uninstall it simply remove gdb from /usr/local/bin/ by executing

sudo rm /usr/local/bin/gdb