I have used command-line based 68K Assemblers on Amiga, but not GUI-based. However, there's a MPDOS Pro which you can try which lets you compile 68K code on a PC and it has the editor and GUI built-in:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320210504317It compiles the source directly into ADF or binary and includes some source code. If you use it with the simulator cable, you can boot your amiga directly with the code or otherwise you can transfer it over to your amiga via some utility. If you are using an A1000, you can compile your own (custom) Kickstart.
