Maybe I'm talking BS here, but I would be wary of using that substance as a catalyst. Formates can work as hydrogen donors under certain conditions (especially CTH ones 8) ), thus forming oximes and methylamine as a byproduct. I don't know if this is a factor that will influence your yields. But better save your formates for proper uses ;)
If you do now have access to pure CH3COONH4, why not generate it in situ? You can use an equal (equal to the amount of ammoniumacetate you would have used) molar amount of ammonia in an excess of GAA. GAA even tends to boost the yields in this reaction. I don't think the slight extra water from the ammonia will hurt your yields (see this (https://www.thevespiary.org/rhodium/Rhodium/chemistry/4f-p2np.html)
(https://www.thevespiary.org/rhodium/Rhodium/chemistry/4f-p2np.html) document for the effects of less water on the knoevangel condensation.
Doing it this way will save your nitromethane (1.1 eq. is enough, the GAA will solvate the rest), boost your yields and give a faster reaction.
Regards
Bandil