Houserule out the "Control" aspect from the spell.
The problem with the spell isn't so much that it calls in powerful monsters, in my experience. It's that it instantly gives you powerful but obedient slaves.
Calling in a Solar or similarly powerful creature isn't something I would like to cut from my D&D experience. What I would like to cut out is that any random Wizard with 9th-level spells can call in a Solar, tell it to make a sandwich, and it goes "yes master". There's no defense against this mind-control for the Solar, no saving throw, no action requirement, just control by the caster as soon as the creature is through the Gate. And that allows shenanigans, increasing the caster's ability scores being the least of which.
Now, if that control were removed, it would remain a powerful and supremely useful spell. You can still call in a Solar when your party is fighting a Balor, I'm sure the celestial would be thrilled to join the fight. You could call on it for advice. You could even ask it to increase your ability scores, which it might, could you convince it your being slightly more intelligent/strong/quick/whatever is a worthy use of its Wish ability. You just can't make it do things it ordinarily wouldn't do, by virtue of the Gate alone.
As a bonus, I'd say the spell now has far more narrative potential. "YOU DARE SUMMON ME, MORTAL?"
Let the Players Help You
If this is early in your DMing career, it's okay to ask the players for help. You've already put the spells on cards, so when a creature casts a spell just flip the card into the middle of the table and say, "I'm pretty sure this is the spell the creature would cast, but I'm not exactly sure what happens."
In other words, give yourself a way to take back the spell if it does something you totally didn't expect, like color spray's weird area stunning all the caster's allies or the obscuring mist preventing the caster from using his sneak attack ability. Do this because (I hope, anyway) you'd grant the same leeway to the new players; while the PCs understand the area of a color spray and that obscuring mist often grants concealment, the players don't yet. But they will.
And that's kind of important. Sharing this information can pause the game, but when running a new system, pauses are okay, and mastering the low-level spells makes using higher-level spells much easier. Further, the newer players also learn how the spells function, and the experienced players learn the kinds of judgment calls you make.
The great thing about doing this now with, I hope, low-level spells, is that those spells will happen again--in later campaigns. Low-level spells like color spray, grease, obscuring mist, shield, sleep, and the like are things your group should agree on because campaigns start with those available. Making sure everybody agrees on how they work is a priority for your gaming environment, now and in the furture.
Best Answer
Freedom of Movement
The spell freedom of movement protects the subject from magical spells and other effects that would normally prevent or impede their movement, which includes the paralyzed condition as inflicted by the spell hold person. Cast in advance, it will protect you from being the victim of such a spell.
More permanently, you could also acquire a magic item such as a ring of freedom of movement, which permanently grants the wearer the benefit of the aforementioned spell. It is a quite costly item, though, and usually beyond the means of characters at lower levels.