How to use a logical address with an FS or GS base in gdb?

how can i read the memory at "%gs:0x14" in gdb

You can't: there is no way for GDB to know how the segment to which %gs refers to has been set up.

or translate this logical address to a linear address that i could use in x command

Again, you can't do this in general. However, you appear to be on 32-bit x86 Linux, and there you can do that -- the %gs is set up to point to the thread descriptor via set_thread_area system call.

You can do catch syscall set_thread_area in GDB, and examine the parameters (each thread will have one such call). The code to actually do that is here. Once you know how %gs has been set up, just add 0x14 to the base_addr, and you are done.

As answered in Using GDB to read MSRs, this is possible with gdb 8, using the registers $fs_base and $gs_base.