SM:
I think some of the reasoning behind you not being awarded a Gulf medal is an AGAI that prevents being awarded 2 medals for the same operation. The fact that you were deployed on a NATO operation in support of coalition operations means that your NATO deployment may have prevented you from also being awarded a Gulf medal irrespective of your actual role. If there was no award for Op Anchor Guard then you would not be entitled to "claim" a Gulf medal in lieu.
A number of soldiers have been caught by this because they deployed with NATO forces in support of UN troops in the former Yugoslavia. Although many of the soldiers deployed with NATO worked alongside the support elements of the peace-keeping troops they were not entitled to claim the UN Bosnia medal and clasp. NATO did not authorise the striking of a medal for work in support of the UN, and as the deployments were under the banner of the UN there was no entitlement to either the issue of the GSM or a clasp if the GSM had already been awarded.
I agree with you tho, you do the time in theatre, you should be able to claim the medal.