The code to merge GIF in C++

using namespace Aspose::Words;

std::vector<String> fileNames { u"Input1.gif", u"Input2.gif" };

auto doc = MakeObject<Document>();
auto builder = MakeObject<DocumentBuilder>(doc);

std::vector<SharedPtr<Shape>> shapes;
for (const auto& fileName : fileNames)
{
    auto shape = builder->InsertImage(fileName);
    shapes.push_back(shape);
}

// Calculate the maximum width and height and update page settings 
// to crop the document to fit the size of the pictures.
auto maxWidth = *std::max_element(shapes.begin(), shapes.end(),
    [](auto lhs, auto rhs)  return lhs->get_Width() < rhs->get_Width(); );
double maxHeight =
    std::accumulate(shapes.begin(), shapes.end(), 1.0,
        [](double result, auto shape)  return result + shape->get_Height(); );

auto pageSetup = builder->get_PageSetup();
pageSetup->set_PageWidth(maxWidth->get_Width());
pageSetup->set_PageHeight(maxHeight);
pageSetup->set_TopMargin(0);
pageSetup->set_LeftMargin(0);
pageSetup->set_BottomMargin(0);
pageSetup->set_RightMargin(0);

doc->Save(u"Output.gif");