Signed languages have a complex visual grammar. Words are inflected in many ways. I am a somewhat native speaker of the local signed language, and I sometimes discover that I uttered something I couldn't translate into German (the local oral language) without losing some meaning. This usually concerns dynamics, location and emotion. One example, I explained my son how to use the chairlift and how the chairlift will take off.
Linguists have discovered that this is not pantomime. I forgot how they named the complicated movements but I remember it is something like a cluster of micro utterances.
Your example of «I love you» is an example of inflection. You can vary this sign, to indicate a special dynamic, for example «for a long time», «slowly increasing in intensity», «and you, too, sincerely and deeply», and many others.
Here is a recent article that covers this in significant depth: https://www.glossa-journal.org/articles/10.5334/gjgl.499/
The emergence of Nicaraguan Sign Language is a very revealing case is this regard: in much the same process as creolization, a sort of "pidgin" of home signs was transformed into a full sign language in a generation by "grammaticalizing" gestures and simple home signs from older children by the younger children. So it doesn't take long for a gesture to move right on Kendon's continuum until its fully "linguistic".
https://en.wikipedia.org/wiki/Nicaraguan_Sign_Language
https://en.wikipedia.org/wiki/Home_sign
https://en.wikipedia.org/wiki/Pidgin
This isn't unusual. Even in spoken languages the required grammar are often different. Even in English some areas there are some contexts where "the" is mandatory in some geographical areas and not others. Linguists can give you a Phd if you want to study this more.
You can't relate the two languages like this. Sorry, this doesn't make sense. For example, ASL doesn't have articles, English doesn't have location dependent pronouns for people.
That's why there are dozens of journals in things like linguistic typology and language variation and change. There are linguists whose entire careers have been generating a sort of conceptual mapping between the morphology, syntax and phonology of spoken languages to sign languages so that you can have sensible conversations about the typological relationship between a signed language and a spoken one.
See the research of eg Diane Brentari at Chicago or Diane Lillo-Martin at UConn or Wendy Sandler at UT Austin.
You look surprised => You look surprise
They came here recently => Recent they come
It's not about the order of words but that grandparent seems to have misunderstood something about glosses. He is thinking that grammar is simplified. It is difficult for written glosses to express the complete grammar spectrum of Signed Languages.
It's unfortunate and similar to the Blub paradox: some people will unavoidably believe that Signed Languages have a simple grammar similar to Chinese when looking at glosses.
Additionally, my local Signed Language (Swiss German Sign Language) has quite a free order. Often topic-comment is applied. An example:
[Concerning the] weather, [I] think [it's] quite fine.
As glosses: WEATHER THINK GOOD.
The word «quite» is omitted because it's an inflection. In SGSL I would make a slightly kissing mouth, lower the eyelids somewhat and make very small and fast nodding movements.
That's not Subject-Object-Verb order.
Additionally there is a slight shift between WEATHER and THINK, it's difficult to explain, but perhaps a colon after WEATHER would help. WEATHER: THINK FINE. This what I called «shift» is an inflection: it puts the word WEATHER in topic mode.
Caveat emptor: I am not a linguist, but always have been very interested in linguistics and talk SGSL every day.
Edit: many little things.
I appreciate your detailed response though.
As a point of note, Makaton is used at least a little at a lot of primary schools in Scotland (I would guess across the rest of the UK too), as a "supplementary language" for all kids.
Like "I love you" as [love sign] -> [point at person you love].
Or for example that "very" might exist in a signed language, but isn't often used. Just substituted with a more physically animated/vigorous version of the adjective, or added facial expression.