Can the G or D strings be muted by Cellist so that a double stop can be played on the G and A, or C and D strings? Would greatly appreciate a response.
No, this doesn't work, at least not for sustained notes. There's no way to “bridge over” the D string when bowing the G and A string; in fact the only way to bow both G and A string simultaneously is to put a lot of pressure on the D-string so the bridge arcing is “surpressed”. If you bow a muted D-string this way, it creates a very ugly scraping noise.
What is possible is to arpeggiate a chord from the G to the A-string, immediately lifting the bow, but mute the D string. This works much the same way as on guitar except the bow is used for “plucking” the strings. The muted D will still cause a bit of a rasp, but it's so short that it may be musically acceptable.
For properly doing wide double-stops, you should make sure they are on neighbouring strings. If the lower note is on an empty string, this is no problem. Using thumb position, it is also possible to play any octaves or even ninths on neighbouring strings, but it can be tricky to get the intonation right.
Alternatively, find a note that can be played on the D-string and add it to the chord. Triple stops or even four-note chords are found all across the literature. Again it's usually understood that not all the notes are sustained – often, only the top two notes are held. For triple stops it is actually possible to bow all three strings at the same time, but the required pressure still makes it sound pretty violent, like in this example from the Henze Cello solo Serenade:
X:1 L:1/16 M: K: T:V Q: "Vivace" %%score B V:B clef=bass % 1 [V:B] ._B,,.F,.B,.G,.^f,2 ._B,,.F,.B,.G,.^f,2 | ._B,,.F,.B,.G,.^f,2 | .^c2.D2.A,2 _A,,6 | .[E,C_E]2.[E,C_E]2.[E,C_E]2 .[E,C_E]2.[E,C_E]2.[E,C_E]2