I solve this problem adding the following line in /etc/apt/sources.list:

deb http://ftp.de.debian.org/debian sid main

After, install gcc-multilib with apt:

#apt install gcc-multilib

I managed to install gcc-multilib but with a downgrade for some components. Don't know if it is a good solution but it worked.

My solution was to install gcc-5-multilib and press n when aptitude asked for the first time "Accept this solution?" and Y on the second time. Output:

sudo aptitude install gcc-5-multilib
...
The following actions will resolve these dependencies:

      Keep the following packages at their current version:
1)      gcc-5-multilib [Not Installed]                     
2)      gcc-multilib [Not Installed]                       
3)      lib32gcc-5-dev [Not Installed]                     
4)      libx32asan2 [Not Installed]                        
5)      libx32cilkrts5 [Not Installed]                     
6)      libx32gcc-5-dev [Not Installed]                    
7)      libx32gcc1 [Not Installed]                         
8)      libx32stdc++6 [Not Installed]                      
9)      libx32ubsan0 [Not Installed]                       

      Leave the following dependencies unresolved:         
10)     libc6-dev-i386 recommends gcc-multilib             


Accept this solution? [Y/n/q/?] n
The following actions will resolve these dependencies:

     Remove the following packages:                                       
1)     gcc-6-base:i386                                                    

     Keep the following packages at their current version:                
2)     gcc-multilib [Not Installed]                                       

     Downgrade the following packages:                                    
3)     gcc-6-base [6.3.0-18ubuntu2~16.04 (now) -> 6.0.1-0ubuntu1 (xenial)]

     Leave the following dependencies unresolved:                         
4)     libc6-dev-i386 recommends gcc-multilib                             


Accept this solution? [Y/n/q/?] Y
The following packages will be DOWNGRADED:
  gcc-6-base 
The following NEW packages will be installed:
  gcc-5-multilib lib32asan2{a} lib32atomic1{a} lib32cilkrts5{a} lib32gcc-5-dev{a} lib32gomp1{a} lib32itm1{a} lib32mpx0{a} lib32quadmath0{a} lib32ubsan0{a} libc6-dev-i386{a} libc6-dev-x32{a} 
  libc6-x32{a} libx32asan2{a} libx32atomic1{a} libx32cilkrts5{a} libx32gcc-5-dev{a} libx32gcc1{a} libx32gomp1{a} libx32itm1{a} libx32quadmath0{a} libx32stdc++6{a} libx32ubsan0{a} 
The following packages will be REMOVED:
  gcc-6-base:i386{a} 
The following packages are RECOMMENDED but will NOT be installed:
  gcc-multilib 
0 packages upgraded, 23 newly installed, 1 downgraded, 1 to remove and 0 not upgraded.
Need to get 11,1 MB of archives. After unpacking 46,1 MB will be used.
Do you want to continue? [Y/n/?] Y
...

And afterwards I was able to install gcc-multilib:

sudo aptitude install gcc-multilib
The following NEW packages will be installed:
  gcc-multilib{b} 
0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 1.212 B of archives. After unpacking 8.192 B will be used.
The following packages have unmet dependencies:
 gcc-5-aarch64-linux-gnu : Conflicts: gcc-multilib but 4:5.3.1-1ubuntu1 is to be installed.
 gcc-multilib : Conflicts: gcc-5-aarch64-linux-gnu but 5.4.0-6ubuntu1~16.04.9cross1 is installed.
                Conflicts: gcc-5-aarch64-linux-gnu:i386 but it is not going to be installed.
The following actions will resolve these dependencies:

     Remove the following packages:
1)     gcc-5-aarch64-linux-gnu     
2)     gcc-aarch64-linux-gnu       



Accept this solution? [Y/n/q/?] Y
The following NEW packages will be installed:
  gcc-multilib 
The following packages will be REMOVED:
  binutils-aarch64-linux-gnu{u} cpp-5-aarch64-linux-gnu{u} cpp-aarch64-linux-gnu{u} gcc-5-aarch64-linux-gnu{a} gcc-5-aarch64-linux-gnu-base{u} gcc-5-cross-base{u} gcc-aarch64-linux-gnu{a} 
  libasan2-arm64-cross{u} libatomic1-arm64-cross{u} libc6-arm64-cross{u} libc6-dev-arm64-cross{u} libgcc-5-dev-arm64-cross{u} libgcc1-arm64-cross{u} libgomp1-arm64-cross{u} libitm1-arm64-cross{u} 
  libstdc++6-arm64-cross{u} libubsan0-arm64-cross{u} linux-libc-dev-arm64-cross{u} 
0 packages upgraded, 1 newly installed, 18 to remove and 0 not upgraded.
Need to get 1.212 B of archives. After unpacking 60,8 MB will be freed.
Do you want to continue? [Y/n/?] Y

I do not often use aptitude, but in case of problems with different gcc versions and unmet dependencies, apt-get is some times unable to correct problems saying "you have held broken packages"

That's the day, you should install aptitude, and try to install the package you wish. It clearly shows all dependencies and makes not only one suggestion.

First suggestions is often the same apt-get would do, but if you click "no" aptitude will show you another one, which maybe fits your needs better.

Give it a try!

Answer is a little late, but I often searched for problems like these and was happy about different solutions.

gcc-4.5 or g++4.5 is not in most Ubuntu software distributions. If you are trying to get exactly this version, see gcc releases and successful builds.

Another option is to use “PPA for Ubuntu Toolchain Uploads (restricted)” team's edition. In your case, add

deb http://ppa.launchpad.net/ubuntu-toolchain-r/ppa/ubuntu trusty main 
deb-src http://ppa.launchpad.net/ubuntu-toolchain-r/ppa/ubuntu trusty main

to your source list, and run sudo apt-get update. You'll be able to use sudo apt-get install gcc-4.5.