Obviously, he doesn't have it in him. Was he ever shown to be able to do this? Claiming that his father had that power and logically Gray has it too, is just like saying Natsu has access to every possible spell Igneel had. If it hasn't been shown in the manga, it's fan-fiction and theories.
Pay homage to the magic of his master? You just agreed a post ago that Gray was fighting seriously and he even said (more than once) that was
NOT Ul, so why in the world would he pay homage to her magic? Also, you obviously missed the part where the spell he used to defeat Ul (along with Lyon) was an extension of his Ice Devil magic, something that was not taught to him by Ul, so I don't see how paying homage to her magic seemed like a valid argument to begin with.
If Gray is going to reach his full power and potential, it's through Ice Devil Slayer magic, and none of the spells you listed from his repertoire are part of it.
So, again, judging the fact that Gray
AND Lyon had a hard time with Ul alone, I don't think Gray has it in him to singlehandedly take on a Spriggan. But we'll see, I guess.
Emotional fight? Where did Gray even get emotional in that fight? His resolve to fight her was definitely close to Erza's, but in his case, it wasn't enough. And are you really comparing severly injured Erza taking on three S-class opponents at once, one of which who's probably on the same level as Ul or possibly higher? Of course she'd lose. It's 3v1, whereas Gray & Lyon was 2v1. I'll let you do the math.
So far, Gray's performance was pretty underwhelming. His magic barely stopped a
Spriggan, who doesn't even control Ice... So let's not get carried away, here. Gray has yet to show us his one-year training.
You know this argument is pretty ridiculous when you're throwing out objectivity out the window and not even considering how characters have options to what they can do. Gray has that power, because it was transferred to him , it doesn't take a rocket scientist to figure something so simple.

Every slayer has the ability to consume their own elements, you're contradicting the own manga with your fallacious claims. It's not "fanfiction" or "theories", it's called reasonable deduction and adhering to logic. There's a difference between using spells, and consuming your own natural element. Take that into consideration next time you want to post instead of downplaying.
Gray wasn't fighting all that seriously to begin with. He was only serious in dialogue and facial expressions but not in his actions considering his position and abilities being used don't reflect his temperament. He knows it's not Ul, but what difference does that make? Gray resorted to Ice Make because he didn't want to expend his magical energy considering there will be enemies he'll be fighting later. It's a common trait for fighters to not go all out from the start.
Even then, he held back his power in his base mode. He didn't even charge fully at Ul, he didnt' release the destruction and AOE he had in the avatar against fodder than he did against Ul. I'll let you know in a secret, Gray has a lot more stronger spells in his moveset than IM Geyser. The same IM Geyser that was stopped by BOS Lyon. All Gray used was nameless spells and that one.
Had Lyon had the same motivation that Gray did, Ul would've been safely done from the start, but Mashima wanted to hype up Ul by handicapping Gray even his base mode, and to provide development for Lyon's character.
Considering Ul's lack of durability, it's safe that IM Gungnir is capable of taking her out. Considering it took out Ultear who has durability feats compared to Ul, and this Gungnir is logically much stronger than the one Gray used on Ultear with SO+ 1 yr of training.
Gray can reach his full potential and full power in his DeS mode that's true, but it doesn't change the fact that Gray didn't use his full potential and full power even in his base mode.
You're judging on a character who was handicapped from using his full reserves, and still was enough to be in fine shape in the aftermath of it. Basing it off as it's some sort of test for what Gray will fare against a spriggan is a fallacy, and one of the ridiculous arguments I have ever heard in my time here.

So congrats on that one. Alvarez Princess nailed it perfectly, this was a mental mind game, and a psychological fight for development, not an actual real fight.
All of the historia's battles that took place was a classification for how strong your mental mind is. If you can overcome the mental hurdle you can surely win the battle. How do you think grievously injured characters were able to oneshot their opponents? Erza's mental mind was so strong she looked them all in the eye and made them vanish from it. It's all in the mind. The stronger your mind, the weaker the historia's become. Love how you constantly neglect and ignore this battle with Ur was a lot more personal than the other historia fights. Whatever fits your narrative I guess. They can't even hold a candle in comparison to this one.
You don't believe DF Wendy who had a broken ankle can actually oneshot Tartaros Ezel. Do you? Good grief.
You mean the same spriggan, that dominated and low diffed Erza? Funny enough people put Erza in a battle against Irene and expect her to win when she got her ass kicked by a much inferior spriggan.

Gray was aiming for the golems ftr to provide a distraction so Erza can wheel it out of there. Gray's motive was to escape by the orders of his guild master.