As Lorena has noted, the relevant meaning of "factura" here is DLE definition 2: "Modo de estar hecho o ejecutado algo. U. m. en arte y artesanía. Retrato de buena factura."
http://dle.rae.es/?id=HTrRse1

I think "execution" would be suitable here, but you could argue for "style" or perhaps "rendering". Both of those would be defensible alternatives in my opinion.

"Gestual" is a standard modern art term and the English equivalent is "gestural". It mainly implies vigorous application of paint and was originally associated with the American Abstract Expressionists:

"En 1950, el filósofo y crítico de arte estadounidense Harold Rosenberg (1906-1978) utiliza el término action paintig para describir la primera corriente artística del expresionismo abstracto de un sector particular de la Escuela de Nueva York y que forma parte de la poética del arte informal, con una posición de vanguardia en la propia Europa. En este movimiento, la acción, el acto de pintar (arte gestual) es de importancia fundamental, y no debe interpretarse como un simple gesto automático, sino como la extensión directa sobre el lienzo de la experiencia del artista. Entre los principales representantes de la action paintig, hay que mencionar a Jackson Pollock, que favorece la técnica del goteo “dripping” (proyección de gotas de pinturas sobre lienzo); William de Kooning, que retoma la violencia colorista de los expresionistas (de ahí el término de expresionismo abstracto), Franz Kline y Robert Motherwell."
https://www.aparences.net/es/periodos/arte-contemporaneo/exp...

"Gestural is a term used to describe the application of paint in free sweeping gestures with a brush.
The term originally came into use to describe the painting of the abstract expressionist artists Jackson Pollock, Willem de Kooning, Franz Kline, Robert Motherwell, Hans Hofmann and others (also referred to as action painters). [...] The idea was that the artist would physically act out his inner impulses, and that something of his emotion or state of mind would be read by the viewer in the resulting paint marks."
http://www.tate.org.uk/art/art-terms/g/gestural