I use MacX1.5 for almost all of my daily work, which is largely on a
DEC Alpha machine. I do not use seqlab normally, but after reading
this thread, I started it up with no problems (a menu font was missing,
but everything started). I was also able to startup on an SGI system.
I recommend you start an xterm from MacX, make certain you can start
other X programs from the xterm, and then start running GCG from
within the xterm, finally typing seqlab.