If you want to write some comments in your GNS3 baseconfig files for the Cisco IOS, you can use the exclamation mark. It is mainly used for separating commands for easier reading. Everything that is written after the exclamation mark is ignored.

Here's an example:

enable ! a comment
configure terminal
interface FastEthernet0/0 ! now I will configure the FastEthernet0/0 interface
 ip address 192.168.1.1 255.255.255.0
 no shut
!
end

Some links:

Cisco IOS Command Line Interface Tutorial