2007 iMac dual screen issues

I'm new to this forum Im looking for help with my iMac 2007 model. I was given a gateway tft19w80ps+ monitor and I used it with an old macbook pro 2006 and it works fine (mothers Mac) and I own a 2007 imac I bought a Mini dvi to dvi adapter and hooked the gateway to my imac and nothing happened same cable as I used with the mackbook pro so I know the cable works I tried an smc and pram reset but nothing got fixed. I reinstalled mountain lion still no luck nothing worked I'm at a lose I need help. The imac shows it sees the gateway just no picture shows on the gateway monitor except for the no signal keeps flashing on it randomly from time to time. I tried calling apple but they are going to charge me for phone support. Grrr not liking the results help of any kind would be apreciated. I did an SMC and PRAM reset. something I'm missing.

The 2006 MacBook Pro models used a dual link DVI output. How did you connect the Gateway monitor to your mother's MacBook Pro and make it work?

Your 2007 iMac uses a Mini DVI output which is different than that of the MacBook Pro. That suggests that either the Mini DVI adapter or cable may be bad. Where did you buy the adapter and cable from?

I got the mini dvi cable from best buy I exchanged the adapter twice. .I hook up to the makbook pro with dvi

Again, if the Gateway monitor worked OK with your mother's MacBook Pro and doesn't work with your iMac, that either suggests bad cables or the video output from your iMac isn't working. And since you swapped out the adapter twice, that leaves the iMac.