Annotating gene names or gene IDs to a dataframe containing SNPs?

I assume your SNP file contains genomic positions - please clarify if not. For example, you could have an rsID column, chromsome, and position column that contain "rs3088379", "chr1", "54225281".

Approach #1: use R, as you suggest.

ensembldb is an R package that may be what you're looking for: https://bioconductor.org/packages/release/bioc/html/ensembldb.html.

You could map the genomic positions of your SNPs to gene names by adapting their workflow: https://bioconductor.org/packages/release/bioc/vignettes/ensembldb/inst/doc/coordinate-mapping.html. After you've mapped the genomic position, you could filter for your gene name.

Approach #2: use bedtools.

You can convert your Excel file of genomic positions to a tab-delimited BED format. https://bedtools.readthedocs.io/en/latest/content/overview.html has an overview. So, your first SNP would turn into

chr1 54225280 54225281 rs3088379

--> make sure you turn 1-based to 0-based coordinates! You could then download the coordinates of your gene in BED format from your favorite source, like UCSC Table Browser.

Then, you run bedtools intersect with your SNP BED and your gene BED. What remains is just that gene's SNPs.