Yes, if you write a program in GX Developer and downloaded it in the PLC, you can upload it in any software you wish.
The only problem that could occur is that you get the program in Instruction list. This will happen if you do the upload in Melsec Meodc + vx.x or GX-IEC Developer vx.x.
In GPP Win, FX-WIN, melsec medoc, GX Developer or FX Developer you upload the program in Ladder. Regardless in witch software you wrote the program and downloaded it.
Even if you write a program in GX-IEC v4.0 and you loose the source code, the upload you make in GX-IEC is in instruction list. (behalve when you download the source code with the program in the PLC memory)
kind regards,
Frank de Jong