Your question makes no sense, what does a GUI have to do with an assembler?
Anyway, what you're probably looking for is GNU.
How can we wrote programs for ARM LPC2148 and program our controller..
Are there any software like Keil for Ubuntu 12.04..
Yes, you can install the Eclipse IDE and ARM-GCC toolchain which provides C/C++ and Assembler development, example:
Using Eclipse to Cross-compile Applications for Embedded Systems
@sky_123 - Code Blocks is an open source IDE for C/C++ development: